OP: if you mean while you are creating caves, yes, that would be a neat option. Once you have your caves, easy enough to drop lights in place, which is more or less what you are asking for? Could just be a simple toggle to 'brighten' the view while using deform tools underground?
On this same topic(well not really), how about peeps being able to buy all those cool flashlight toys and spiny things the theme parks sell before parades and such? This might be a substantial drain on CPU/GPU resources though. Random peeps with those fiber optic spin toys, or various similar options sold from vendor carts? At least this way, it could tie into the 'theme' aspect of the game. I think peeps with bright shinning flashlights might be too much though. Just a dim, ambient glow would be perfect [happy]