To my question:
Let's say an enterprising goetic sorcerer ablates a Famulus specifically to gain the power Shroud the Stench of the Pit....
how should that be adjudicated?
My temptation is to convert the ablation points into XP in a new ability called Shroud the Stench of the Pit and use that score to cast some non-hermetic variant of Shroud Magic at that magnitude.
Are there any immediately obvious pitfalls to doing it that way?
I would use the rules from RoP:M for creature powers used by beings without Might.
See for example the virtues Greater Power and Ritual Power in RoP:M p44,47
Shroud the Stench of the Pit
Init equal to (Qik-2), MuVi 15, Shroud the infernal taint of the users magic up to 6th magnitude. (Personal Power: Two Mastery points spent to reduce might point cost to zero, Constant effect)
Or this?
Shroud the Stench of the Pit
Init equal to (Qik-2), MuVi 35, Shroud the infernal taint of the users magic up to 14th magnitude. (Greater Power: Three Mastery points spent to reduce might point cost to zero, Constant effect)