One thing that I am thinking of doing is this:
Children born of ONE parent who is Imperial (by Imperial, I mean Imperial Citizen), can not be deemed to be "Citizens". Otherwise, the Imperial War Machine will have a problem getting recruits. Why? Once a soldier gains Citizenship, when they return home and marry, that results in the children being deemed "Imperial Citizens". As such, if Imperial Laws are such that Citizens gain certain protections under the law, then treating "Citizenship" as if it were a "Dominant" Gene would show that the following would be true:
Only children born to Imperial Subjects remain subjects. If both parents are Imperials, then child will be Imperial. If One Parent is Imperial, then child is Imperial. It would take some time, but LESS time than 1,000 years for a population to be deemed "Imperial Citizens".
It would also add to the court cases, those instances where a child states they are children of Imperial Citizens (Former Soldiers) and are entitled to the protections of the Imperial citizen's based laws. Sort of like trying to prove you're a free man in Medieval times.
It might be possible to state that if the Mother is Imperial, the child born to her will also be Imperial rather than a subject, because a Mother's status as the child's parent can not be contested if the child was born of natural child birth.
Now, tossing further thoughts out there...
If you're born an Imperial Citizen, one of the rights of an Imperial citizen is that you can only be tried for certain "issues" in an Imperial Court. If in a court case, all things being equal, one is an Imperial Subject, the other, and Imperial Citizen, the Imperial Courts might automatically decide in favor of the Citizen.
In short, there will need to be some reason why people WANT the Imperial Culture and that it be an advantage to live by its codes. Now, Knights have a new responsibility if you will...
1) they're the legal representative of the Iridium Throne when they are the only one present.
2) They're required to observe the world government to insure that the Government pays its "Tithes" or tax burdens.
3) They're required to administer to the Legal machine of the Third Imperium as far as courts, tax rolls, census taking, etc.
4) They're required to certify that the world does not practice Slavery
5) They're required to certify that the world's government does not trample the rights of Imperial Subjects as determined to be a minimum required legal standing
6) They're required to exemplify the proper conduct of Imperial Citizens.
7) They're required to patronize those artists who maintain Imperial Culture within their media (Music, Performing Arts, Sculptures, Movies, etc)
8) They're required to seek the means by which the world advances and becomes better integrated within Imperial Society
and the last one is the most important one...
9) insure that they collect the dividends of the 2% stock the Imperial Throne gains for its ownership in all public stock.
Any and all of the 9 aspects above may be the domain of Nobles on any given world that has them, but on worlds that do not have them, the Knight is the lowest possible representative charged with handling these things.
Eventually, as worlds become more populated, and there are Barons and the like, eventually, the structure for the Subsector Representatives for the Iridium Throne will become self-funding and self-governing. The subsector Dukes respond to events in their area in a manner that would be deemed "approved by the Throne". Having the immediate ability to respond to events is their first job, and anything else is simply "power grabbing" at its best/worst.
Take for example my own Lunion Subsector campaign. I have a Baron of Lunion whose last name is Robertson (old Solomani Family that migrated out to the Spinward Marches back when the Solomani Star was seen to be rising in the court of the Emperor). This family has a son who stayed home helping his father the Baron - administer to trade relations with various worlds at the direction of the Duke of Lunion. The son named Peter Robertson, fell in love with a common born woman who was a nurse serving on Adibicci. (I'm using GURPS TRAVELLER by the by) and the world was described as being heavily repressive in a fashion not unlike the Soviet Union was reported to be. So, I ran the Adibicci world as one that watches over its citizens in a repressive fashion, with off-worlders required to have a guide who helps them through society. Those who live under that regime would love to see it replaced with someone less oppressive, but don't have the means to do so, short of off-world help. Enter the Baron of Lunion's private machinations. They are funneling money through secretive means so that the money can't be traced to them. In addition, they ship chip adapters that use TL 15 chips in TL 13 goods. These adapters were also "re-purposed" by the Lunion Organized Crime cartel for their own nefarious purposes. But long story short - by allowing the adapters to be sold, the citizens of Adibicci can create some pretty secure communications devices that the Authorities on Adibicci can't hack or eavesdrop on.
As a consequence of all this, the fact that the Son married a commoner is deemed to be a bad thing in some social circles, and the fact that the powers that be in the government who control ASP (Adibicci Secret Police) believe the Robertsons behind their problems but can't prove it in a court of law - have undertaken to attempt an assassination not only against the Baron himself, but against his son.
Once the assassination attempts were discovered (one by means of a gauss Rifle 13.5mm anti-vehicle weapon, and the other by means of Akaloid poisoning) - the Imperial Marines were handed a mission. They were to infiltrate each of the Industrial Magnates households on Adibicci, leave a dead animal's head in the room of the individual in question - and leave without inflicting casualties. It was an exercise designed to intimidate those malefactors into understanding that should the Imperium feel it necessary, they can easily take out heads of state as well as heads of industry. That got the message across until one group attempted to "Terrorist" attack against the son while he was aboard a Empress Marava Model B ship. This in turn, resulted in the Starport being closed for a period of 4 months as a "condition Red" Threat assessment.
Now - when it comes to doing things behind the scenes, the rule is "Do not get caught". But when a government violates the rights of the subjects to the Iridium Throne, the Nobility are expected to handle it. They are also expected to be diplomatic where possible, but resolute where not. Since nothing is specifically codified one way or another, it keeps the Nobility on their toes so as to not become "Disavowed" or fall into ill-favor, but it also keeps them on their toes looking for ways to earn the approval of the Emperor. Since the Local Subsector Duke is the nearest representative of the Emperor outright, it often means they curry the favor of the local High Nobles.
Now, if I could only figure out whether or not there should be a local "Mini-Moot" gathering at the subsector level. Perhaps the local knights are to the local mini-moot, what the Moot is to the Emperor in some ways. They advise the Duke, and they are his eyes and ears in many things, but they don't ratify anything nor actively create laws (other than for their own personal fiefs).