Go to the hardware store and buy yourself a length of PVC pipe or this plastic pipe that has an internal diameter just slightly bigger than your cable housing. When you get it home cut notches along the length of the pipe but don't cut right through the pipe. Tape this to your handlebar. Then you can feed the cable and housing under this. When it's taped over the cable housing can be removed if the housing needs to be replace yet the bar tape/wrap doesn't need to be removed. Conversely, you can remove the bar tape/wrap without the cable housing being disturbed. There used to be a product made for this exact purpose. I have here somewhere. I tried finding an image of it for you but the beast I can find so far is this drawing.

Shimano Bicycle System Components (1982) page 82

Here's an image from that page.



If I find my pieces of Outer Pad soon I'll edit this post and put an image of it in.

Found them and took images for you.





Do NOT drill your handlebar! Especially, don't drill near the brake lever as many handlebars that were so drilled broke and when they broke they broke suddenly.

Cheers