I'm going to go out on a limb and say that there's nothing official. I've written about the 8th-level Sor/Wiz spell simulacrum [illus] (Player's Handbook 279–80) a few times here, and I've always been impressed by how woefully under-detailed the spell is. I mean, since the DM probably had to make a half dozen or so rulings to get far enough for this to even be a question, expecting the game to have gone even further and expanded the spell's mandate to cover officially this situation is—I'm almost certain—too big of an ask.
However, unofficially, in campaigns that I've DMed I've allowed creatures to add controllers to absolutely loyal minions by extrapolating from the golem rules:
The golem’s creator can order the golem to obey the commands of another person (who might in turn place the golem under someone else’s control, and so on), but the golem’s creator can always resume control over his creation by commanding the golem to obey him alone. (Monster Manual 134)
For example, in my campaigns, all a creator need do is tell the simulacrum Obey this guy, and, forevermore until told by the creator not to, the simulacrum would obey that guy. I've used a similar ruling for handing off undead creatures, called allies, and the like that could receive appropriate commands. (This got complicated enough that I had to rule that an absolutely loyal creature is absolutely loyal to the master's soul, and it can always recognize commands issued by that soul. When its obedience is shared with another, it also pledges obedience to that additional master's soul, and it can always recognize commands issued by that soul, too, and so on. Yeah, cuts down on the impersonate-a-PC shenanigans, but I always hate those anyway.)
Anyway, this has worked, and my campaigns didn't suffer for it. In fact, it made everybody's lives, unlives, and not!lives much easier.
Note: Obviously, attempts to transfer ownership of a homunculus fail spectacularly. I mean, it'll try really hard to obey, but then you'll get to 1,501 ft., and it's outta there.