OK, My suggestion here. First off, I have insulated walls and ran wiring through my spars and then down the inside of the walls.
However, if I was to use solid walls, I think that I would route out a slot for the wiring and put an insert in the slot afterwards, like was suggested by several others here. If you plan ahead well enough, the insert could be a contrasting wood and you could route the wire to make it an interesting design also.
JMOYMMV!
