I will probably regret this, but the problem I have with this is that the zombie can count as a corpse marker for the targeting and/or the resolution according to its ability. If you are counting it as a marker for the resolution, then the poisoning action doesn't do anything, because poison doesn't deal damage to markers, and markers can't be killed.
So I don't think you can suddenly switch the zombie to count as a marker after the damage is dealt.
(this is based on the view that markers don't count as models, and so if you are counting it as a corpse marker you aren't counting it as a model, which isn't expressly stated. I used to use a similar logic that if you were counted as an enemy model you by default weren't friendly, but the FAQ said otherwise in that case, so it could be faulty logic).