As much as this is true, I myself am a technician, and not a smoker, while I don't care what people do to themselves I do have the right to refuse work.
I am able to refuse to work on a computer or any other device that I find to be: unsafe, unsanitary, or a bio hazard. The problem is when the smoke from a cigarette meets with dust going into the computer it will turn in to a really nasty brown gunk that smells absolutely horrible. Now I would recommend that someone have their computer cleaned, by myself or another company before work can be completed on it, now for my work it's only $10 to dust out a computer (plus I get to break out the wicked bio hazard gear).
In terms of going outside of your warranty, I would recommend speaking to apple care and let them know of the situation and that you are going to get it cleaned but it would take you outside the warranty period and if they could open a window of time after the apple care expires, wether it be a week or two, where you can still bring in your computer for work. Opening up the bottom to clean out dust/smoke won't void the warranty, but be sure to document to the apple care team everything you're doing/planning on doing so there aren't any surprises.
As much as it sucks I can sympathize being on the other side of that counter.