Okay, so after reviewing how fronts, threats, and injuries work, I had some ideas that I've yet to consolidate into anything workable, but here they are in case anybody else can build on them better than I can.
Even though we're discussing a 'micro front' based on one character, I think it can be assigned a particular agenda/dark future (i.e. "You go irrevocably insane" or "you are hopelessly lost to Chaos/the dark Fae/the Dark side/whatever). The stakes would all be questions about what will happen to the afflicted person and his relationships ("Will Pitr allow his family to die in order to gain forbidden knowledge?" or "Will the other player characters still trust Gaunt in a crunch?"). I can see affliction threats for specific aspects of the death spiral, and NPC threats for any NPCs actively tempting/corrupting/driving crazy the PC, complete with custom moves to resist their effects.
So far, so good.
Now, as mentioned above, what gets me is tracking the descent, especially in a way that encourages death spiral-ness. Right now I'm thinking of a sort of hybrid of an injury clock and a front clock (works prescriptively and descriptively, 12 o'clock is 'removed from play as a PC', sanity/morality/what have you damage treated like harm that advances the clock). When you take death spiral 'harm', you mark off the clock or mark off a debility (things like phobias or dark auras or such that impair your traits).
What I think is lacking above is sufficient temptation to push the envelope, to be tempted mechanically as well as fictionally. So I was thinking that maybe your insanity or corruption or what not works kind of like Hx. It goes up and down based on fictional circumstances, and it grants you special abilities (insight into things man was not meant to know, dark powers, bloodthirsty rage, et cetera), but when you use it, if you succeed it goes up (or maybe you choose between it going up or some other negative consequence). And maybe every time it goes up to +4, you reset to +1 and go up a tick on the clock (and some especially horrible things would force you to go up a tick if you fail a willpower test or something like that).
Like I said, this is in no way coherent yet, does it sound at all interesting?