This is pretty much the way I've been doing this. The character is in play with gradual effect (20 mins) on his LS Toxin Immunity and LS Disease Immunity, and luckily (other than copious amounts of alcohol consumed) it has never been put to the test. Unfortunately, there are several problems with this:
1) I'm a Poisons/Diseases nitpicker - a degree in Pharmacology does that to you. 20 minutes gradual effect means that he'll see pretty much the full effects of a fast-acting poison (e.g. tubocurarine) but will never even realise he's been poisoned by a slow-acting or cumulative toxin or been infected by a disease. There's not many diseases will kill you, or even show any symptoms, within 20 minutes of exposure. So it just doesn't feel right.
2) There's more than one GM running the campaign. We're running with a round-robin GMing system so there's roughly half-a-dozen GM's to interpret and okay this. It's one of the reasons I was looking for a more concrete way of modelling this power.
3) It may well become an issue at some point in the campaign as at least two other PC's and several NPC's have attacks defined as poisons or diseases (don't even ask!).
Might just do this - buy off the limitations on the powers just to cut down on the potential hassle.
Thanks again.
Smudge