I think Portrait of Ruin handled the "Drain HP" version of poison best, since being poisoned would encourage you to switch characters to avoid extra damage, which at least added a little challenge. But in a game like SoTN, Drain HP poison is technically the same as being damaged normally, except it's like you're being hit repeatedly with weak, but unavoidable attacks that don't make you flinch.
If the drain HP version of poison is to be used in SoTN, it has to be designed in a way that adds challenge. Traditional drain hp poison won't do such.
Either that, or you could stick with the original poison effect in SoTN, which, I agree, does a much better job.