Now that I'm getting used to seeing the scripting (yay!) I'm starting to see what would be needed for a Demon mod: a "Torment" variable. Using the already available "Faith" variable with a Torment one, we'd have the necessary basics for it to work. Although, there should be a "Temporary" and "Permanent" Torment indicator, like Willpower to better keep track of it. I'll see what else there could be for the Apoc Forms as well.
The powers could be relegated to a "Common" slot, an "Apocalyptic Form" slot and three "Lore" slots.
It's not exactly perfect, but it would be damned close (pun somewhat intended)