Deceptive Guise of the Son of Adam
Muto Animal (Corpus) Level 20
R: Personal, D: Sun, T: Individual
This is The Beast Remade (p. 119), with Personal range.
(Base 10, +2 Sun, free requisite)
This spell is fundamentally only useful to Bjornaer magi who, in their Heartbeast form, are affected by Animal rather than Corpus and Mentem (pp. 91-92, 117). Through this spell, magi can overlays a human shape over their Heartbeast form, becoming once again susceptible to Corpus magic, while their mind remains safely out of reach of even the most proficient master of Mentem.
This spell was invented by Blanche of Bjornaer after a near-disastrous Wizard's War where her deficiency in Mentem proved a dangerous liability.
Discussion: the fact that Bjornaer in heartbeast form are unaffected by Corpus and Mentem is clear. However, it is much less clear what forms someone subjected to other shapeshifting effects belongs to. Is a man that has been turned into an animal subject to Corpus, Animal spells or both?
Common sense would indicate that his shape dictates that Animal applies. Furthermore, the Animal guidelines (p. 117) state that if a human body has animal parts (their origin is not stated), those parts are subject to Animal rather than Corpus.
On the other hand, the Limit of Essential Nature asserts (p. 79) that "A things's essential nature cannot itself be changed" (nope, not even temporarily with Muto). Therefore that man is still essentially a man, subject to Corpus. Additionally, it is indicated that shapeshifters can be detected by InAn or InCo magic (q.v. Skinchanger pp. 48-49, Lycanthrope p. 55).
I can reason that an InAn spell would probe the animal shape and detect that it is the result of a spell while an InCo spell would probe the essential nature and figure out "what's really there", but should other techniques be allowed the same distinction? My gut feeling would be that no, they only affect the "effective" form of the target.
Back to the spell itself, I think that the Mentem immunity is sound, but depending on how you treat shapeshifters, you might add a Corpus immunity to it. Or not. Added to that is the question of whether a Bjornaer magus magically turned back into a human can still be considered to be in his heartbeast form or not, which would completely void the spell. Or you might allow the caster to be able to automatically cancel the spell by resuming "true" human shape without requiring a heartbeast roll.
Fun for all ages while waiting for the Mystery Houses book. 
See also this post (and the associated thread).