Drilling a hole and putting an even smaller bolt in there to bleed it doesn't make sense to me. I get the idea like the bleeders on the Chevs but then you will have to worry about an even smaller bolt getting stuck/corroded in there over time too, no? Plus as you mentioned the cap is quite thick, so the bleeder screw/bolt would have to go somewhere else to make it the high point in the system?
I'd take the cap off even if you destroy it and get a new one, or weld a nut to it. Spray it down with penetrating oil etc to get it free. I broke three ratchets taking mine off before I said screw it and took the vice grips to it. I used the old one and welded a big nut to it, and repainted it. Works like a charm now. Do use antiseize like the guys mentioned. I've had mine off a couple times since with no problems now...