There's no problem with drilling the bars, when i have modded bars for others i have milled a slot instead of drilling just a hole say, 10mm by 30-40mm long, its a litte easier to get the cables inside the bar that way.This might sound obvious but... be really careful when deburring the hole/slot so no sharp edges remain, i've come across quite a few H-D with shorts in the electrical system because of this.A simple way to test that there's no sharp edges is to use a paper towel or similar.Put the towel inside the slot, and pull it back out, when the towel doesn't tear up when you pull it out youre done.
No need to worry about the strenght, if you make your own bars, use seamless tubes
(hydraulic tubes work fine) 2-3mm wall thickness is enough.
Just my 2 cents...