I think I remember something like that in Morrowind, where if a target was immune to poison or a certain type of magic, you could make them weak to it, and then use that damage type. For example, if a Draugr is immune to poison, can I use a weakness to poison potion on him, and then poison him normally?

i won't work in skyrim because the only way to deliver the weakness to poison effect is through poison

That svcks in a way, but is understandable. It does seem to limit alchemy in comparison to the other crafting skills though I think.

To answer your question theoretically, a boost is just a weakness in reverse. So, a boost in Posion Resistance can be overcome with a Weakness to Posion. As stated above, the issue is how do you give the enemy the Weakness?

I don't know if the delivery is the issue, as Posion, and any other effect, is really a Magic Effect (affect, duration, magnitude) that affects Health. Resistance to Posion would be just that, Resistance to the Effect which affects Health.