Have to agree with Kramer there, you're bound to forget & switch off the PJ thinking it's a light switch. Not good for PJ to stop without cooling cycle.
As to isolating the wiring, depends how it's been done, you say the switches are standard 2 controlling switches at room ends. I presume by this you mean that both the lights are controlled by each switch. This would not be as straightforward as a way of using one switches wiring to feed a PJ as in effect, the lighting circuitry will pass through both switches, probably switching positive through the single pole/single throw pair of switches. You'd need to gain access to the wiring under the floorboards & have a play with a meter to find out how it's been wired, after that it's a case of a few pence on a junction box & you could isolate one of the switches out of the lighting loop freeing it up to activate a projector. All relatively simple in theory, but as per above, I wouldn't recommend it as eventually someone will hit that switch & shut off the PJ - unless you have some sort of locking or caged switch - not too pleasing on a lounge wall !.
Probably the simplest alternative is to drill a hole through the panel board upstairs & straight down through the ceiling. (tip here is to use a normal bit to go through the chipboard, then use a long (15"+) masonry bit of a smaller diameter (the sort most people have lurking about for drilling through external cavity walls) to go through the plasterboard.) This then gives you a way of routing the power cable up into the room above. Once it's up there, you can run it under a carpet to a nearby socket & hook up via an RF filtered surge protector - Easy !!!
Probably the beat way (at least in my opinion cos' thats how I've done it !) is to bite the bullet about getting acces from the panel boards above & put yourself in some access hatches. I used my router to chop out square holes over my screen & PJ mounts & rebate lips to accept corresponding hatch covers (ensuring they span 2 joists for adequate support). These hatch covers were made & rebated with a router to fit each hole. That way you have relatively painless access from above to fit support brackets, wiring, video cables, etc.
However, if you are in any doubt over any form of electrical work - CALL IN THE PROFESSIONALS !!! And also, if hacking through floorboards be super careful about water pipe layout & buy a pipe detector & treble check before cutting anything !
Even the experienced get lax & make mistakes & trust me, no drama ever comitted to the big screen can ever match the true horror of the involuntary indoor fountain !!!
Sorry if I've rambled on a bit, there's loads of stuff on here from other guys about how they've done installs, have a nose through their threads & you'll soon get some ideas.
Cheers,
Andy