It seems dumb to support *parsing* integers in weird bases, but not *formatting* them in weird bases. Not a big deal, but if you're going to give me a toy, at least give me the whole toy!
The %b idea is a little disappointing in two ways. Even with %b, Python is still dumb by the above criterion. And, I suspect users that don't know about %b are unlikely to find it when they want it. I know I've never looked for it there.
I think a method 5664400.to_base(13) sounds nice.