Id avoid it. I started muay thai with a friend in early 09 and sustained a few injuries which kept me out of the gym. Anything that is going to impede your goals as a BB'r, avoid it. That is, if youre serious. I dont compete but not being able to SFW was depressing, even if it was only for a week or so.
Boxing is good cardio though. If you feel you're losing gains because of all the cals burned, stop...or just up cals. But getting hurt in any sport will leave you out of commision and unable to lift. i understand youre just whacking the bag around, so you should be fine.