Does being poisoned while asleep count as damage for the purposes of waking a character up? My instinct says no, having to make a fort save vs poison will not wake you up. However, I can't seem to find anything conclusive.
Depends on how the poisoned is delivered. If it's through a dagger or knife, sure. If the poison causes convulsions, that's also a yes. Otherwise it's a DM's call.
This one would be the Veneficus Witch's Toxic Words ability, which lets you add a poison to a hex as a rider effect, at the expense of a -2 penalty on the saves for both hex and poison. Mainly assuming via the Slumber hex, to keep "damaging" them via poison without having to worry about the whole waking up thing in the midst of combat due to actual attack damage. .
I would say no, because there are poisons whose effect is making you unconscious (drow poison). If you woke up every time you made a fort save, it would defeat the purpose of those poisons.
Most poisons deal ability damage, which, being damage, would wake a sleeper.