In theory, it's possible to drill holes in the frame for wiring. If you want to do this, get advice from someone very knowledgeable, because you don't want to weaken your frame.
Here is a post about wiring your lights fairly discretely.
I didn't take much care to make my wires invisible, and they're not very conspicuous. The wires are very thin. They are certainly not the first thing you notice on my bikes.