Suffer the Flesh is in Magic of Eberron, p. 103. And I have good news. It is persistable, and it has no language like "this CL increase lasts until the damage is healed" or anything, so it's a go for immunity, or Naberius abuse.
Good find!
EDIT: The exact language is "for each 2 points of damage dealt..." so that definitely works for Naberius. If immunity prevents damage being dealt, then that might not work.