Is it possible to make MuCo changes permanent with Vis (or some other mechanic that I'm missing)?
E.g., my ex Misc may have a possible apprentice, perfect in every way but one: he's a guy. And her Tradition is only passed on to women. Is it possible to use Vis to make a casting of Gift of the New Body (used to turn him into a woman) permanent and thus get around this restriction?
Who is this perfect apprentice? 
Just thinking. Don't think I'd go for Ulrich, but if nobody else wants to teach him...shrug And it's as much a theoretical as anything else. I just thought I remembered something like this in 4e (although I may be mistaken) and was wondering if it were still possible. And, upon checking, it was, in 4e: several of the spells in MuCo (including two that Fiona has) could be made permanent with Vis.
Use a different spell. Use a Perdo Corpus spell!
After all, in Mythic Europe Aristotle is right and he says that women are a lower form of human!
If you want to turn a woman into a man, btw, it's CrCo, because you make her more perfect.
I'm not sure if that is in any way acceptable by the rules, but it's common sense (in 1220).
Sooo...that would be PeCo base 20: "Destroy or sever a limb, so that it cannot heal naturally"? 
Now Ulrich, this won't hurt a bit...
Very last bit on p 79 (ArM5)
so no, unfortunatly not possibly, explicitly forbidden by the Limit of Essential Nature (ArM5, p. 79).
Not using the rules for standard Hermetic magic from this edition. I'm not sure but something from Faerie Magic or a Mystery that might help come close to permanent. You could create a magic item that causes the effect to be continous. Otherwise, the only way would be to find or create a new duration.
In the end, however, you're stuck with the fact that you nearly perfect apprentice is a man and your tradition doesn't allow that. Even if you magically change him into a woman, as Tellus points out, his Essential Nature is male. No matter how you try to disguise the fact of his Nature, training him breaks with your tradition.
Actually, yes. We just have to use the back door. We see in Mages of Hermes a mage who has made a living tree his talisman. Talismans are magic items. So we can make living things magic items. Enchant your Apprentice as a magic item, then add the effect you want, using the rules for continual effects. Ta Da!
Make darn sure that the apprentice in question likes you. Pretty sure that once they pass the gantlet they can bring you before the Tribunal for harming their Gift....
That would not fly. Enchanting a tree and a human being as a magic item are 2 quite different things.
Xavi
The point is that a Muto Corpus effect is something that "overrides" the essential nature of the subject.
This has two consequences. The first is that it will now allow one to go around problems such as:
because you may change the body of the subject into a woman's body, but the subject's essential nature remains male.
The second consequence is that any such change must be constantly sustained by magic. The best way to accomplish it is to bind a familiar, and enchant the effect into the bond. In this way you can make it effectively permanent while avoiding warping.
You'd probably still run head-first into the hermetic limitation refered above.
I'ld say one sure way: enchanted tatoo with one Continous effect.
Well, Xavi, it doesn't seem to break any of the limits of Magic, greater or lesser, and seems to be covered (S.P.) by "Inscription of the Soul" from the Mysteries book.
Can Hermetic magi do tatoo magic?
Yeah, this is, in canon, a problem with permanent sex-change vanilla Hermetic spells.
A Mystery Cult might have a script to change sex, but that's not easily viable for someone who is not yet an apprentice. However, some creature from any of the supernatural realms can probably change a character's sex --- if the characters can convince the creature it is a good idea to do so.
Also perhaps consider whether the tradition is really only open to women. Perhaps, it is only a social thing (rather than magical thing) that only women are accepted into the tradition. So, the potential apprentice might be able to pretend to be a woman, get accepted, and then have to hide his real gender. Maybe his immediate mistress even knows of the deception, but the other members of the tradition don't. Lots of story opportunity there; if you want to go that route.
HoH-S has the numbers and the rules, and i think that there are a few more on TMRE, but yes, any Hermetic magi can enchant tatoos. It isn't totally like the Gruagrachan tattoo Magic but you can make a Invested tatoo and isn't limited to lesser Enchantments.
Wow. Quite surprised you try to pull this here.
IMS it certainly would not fly. A human being is not a magic item. An animal would not be acceptable as a magic item either, but I could be beaten into accepting it in extremely restrictive cases. A human being, no. Otherwise I doubt most covenants would NOT have grogs that can cast POFs "Dragon Ball style" on a regular basis, and his city-minions have ReMe spells enchante din their eyes. You enchant them as minor enchanted devices and there you go.
But well, your saga, your rules 
It breaks suspension of disbelief and the setting for me, so as said, IMS it would not fly.
Xavi
...and by this he means HoH: MC p. 51 (criamon chapter)
and by ths ofcourse he means a different method, allowing the body to be a talisman.
Sorry the error, it is that like you say of course.