I've seen plenty of frames with holes just as I describe for an internally routed rear brake.

Shouldn't be anything close to a concern on a hi-ten frame given the thicker tube walls; cheap frames are often stiffer than nice ones, and I'm sure this one is plenty strong enough. Hell, I've seen the same done to ally bars without issue; that's verging on sketchy.

If I was going to go to all the trouble of breaking out the oxy torch, I'd only bother with a nicer frame, and I'd make it a split housing setup.