Why not look/ask in the NVSE thread, given there's no actual documentation for NVSE yet?
I looked it up on the geck wiki, and it looks like a vanilla function that's been in since Oblivion. There was no mention of OBSE, FOSE, or NVSE.
getAV DamageThreshhold is exactly what I was looking for.
Now, what I'm looking for is, if you shoot someone with AP ammo, which DT will be returned with that function when the function is run on hitting the enemy? Will ammo modifiers be counted first? If so, will it go negative if the DT modifier is greater than the enemy's DT, or will it bottom out at 0?