I can't put my finger on it and it's not OFaolain's reasoning... but something about this feels wrong. Sure, there are times when friendly nobles are wrangled into situations like this, but what binds them to it is politeness and maintaining diplomatic relations, not actual restraints or enforcement. In that case, it's already possible in-game; you just need the right government system. There's also the point that lower ranking people are less likely to have such an offer accepted or are less likely to command enough respect or power to be a big diplomatic deal if their guest leaves early.
For example, if Duke Importantson has a controlling vote in a Senate that will decide if your two realms are going to war, you are more likely to stick around for however long he wants (provided you don't want to fight). If, however, Baron Whocares isn't allowed to vote in the Senate, you're not going to feel particularly pressured to stay... unless he uses armed guards, which now becomes an actual arrest. All but that suddenly hostile arrest are already possible in-game. You just have to start playing that way and maybe toss out a few RPs to make your model clear.
