That is a good idea, was the hole big enough for a grinder to get in there and cut the head off? I know I will be running into this problem when I get enough courage to do this someday soon.
You can drill whatever size hole you want. I drilled 2" holes, which were more than big enough for what I did.
Yeah, I suppose that you could get a dremel in there, but then you'd need to replace both the nut and the bolt, which is discontinued.