I'd suggest against using a standard metal bit as a gag. While I don't have any experience with one, the rubber on our bit gag ends up badly chewed after a decently intense session, and doing that to a metal gag seems like a recipe for a cracked tooth. I don't even want to think of the potential damage if she falls while wearing it.

Also, I've heard that using a snaffle bit is an excellent way to dislocate your sub's jaw, so that gets an extra word of caution.