TheBrain,
I haven't seen an OTU reference.
IMTU, it would depend.
Huscarles could not, under *most* circumstances contravene a world's law level.
The exceptions being:
1)The noble is personally travelling. The noble is entitled to be accompanied by a reasonable retinue of assistants *and* Huscarles, *all* of whom may be reasonably armed. On worlds with law levels high enough to ban the armaments carried, those armaments will have to be concealed, and if they can't be concealed on the body of the Huscarle, wouldn't be acceptable to carry. "Reasonable" numbers is left up the individual GM.
IMTU: If the Sector Duke shows up, "reasonable" for him may be a force the size of the US President's on-the-road Secret Service group; and his influence may even allow for a few light military grav vehicles.
Reasonable is modified when the noble must travel, on official business of the Imperium only, into an active warfare area. Then the Huscarles will be able to dress in outright military gear and carry heavy weaponry. Most nobles dislike this, as unavoidable accidents have a way of happening in such circumstances. If a noble travels into a warfare area on unofficial business, that is the noble's own choice and risk, and no special privileges beyond standard weapons carriage are permitted.
Violations of these laws are handled on a case by case basis, but usually represent a chance for the Member World to extract some concession out of the Imperium (usually tax rebates, bases, x-boat route realignment, etc.) in return for not throwing the "book" at the noble. Imperial Nobles who are "over" the violating noble are likely to be, displeased, at the offender. Major snubs in the noble social world are certain to be applied. Fines are also likely.
2)The Member World is in violation of its Membership Charter and the terms of MTU's Limited Member World Autonomy rules. However, IMTU, it's highly unlikely that any noble with Huscarles sufficient to handling this task would use them in this situation, it's a job for the IN/IM.
3)The noble in question could send the Huscarles in if there were warfare operations underway on the Member World, and in that event the Huscarles would qualify as Mercenary forces. However, if the Huscarles were killed in battle, the noble could publicly say or do nothing, any retaliation would have to be done secretly and never found be out about. If the Huscarles were captured and there were no arrangements for removal, or if the Member World's government wasn't obligated to comply with the removal agreement, at best various Imperial Officials could plead for the lives or release of the Huscarles (depending on their impending fate), try to apply secret pressure, etc., but would have no public direct authority to make the Member World give them up.