Having done something sortof similar, here's my .02 -
first are the lights in the closet incandescent or flourescent? I read it as incand. so here's my idea -
re-wire the existing switch to a ws467 x10 wall switch. (it won't matter anymore how inconvienent it is) Put a motion sensor IN the closet coded to that switch. You want it to work 24x7 not just dusk/dawn, to account for varying light levels in the room outside the closet. Plug in a TM751 (which I suppose you have, you're here on X10 and I assume you already have a little bit of x10 hardware?)
Poof! simple, cheap, done in less than 30 minutes/dollars if you do your shopping right.
You open the door, the light comes on. You close the door, the light goes off in 1 minute (default setting for any x10 motion sensor)
HTH
Tom