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.
This.
Depends on what you want. I boxed for quite a while and also picked up muay thai a couple years back. For the goals you want in both,..... they really dont mix. It depends on how serious you are about both bb'ing and boxing. The cardio can kill your gains in the gym(being high intensity), and the bag training also tends to overtrain your upperbody.
If you are quite adamant about doing both, I would suggest maybe a 3 day w/o routine, with a coupe days in between bag training and shoulder day/back day. This will minimize overtraining, and also do interval training on the bag. You can do both, but dont expect major gains in the gym, as if you where doing no high intensity cardio (boxing) at all.
I go back to boxing during my pre-summer cut, but I also re-arrange my routine so I wont overtrain the upperbody, or legs, for kicks/knee day. And make sure you get PLENTY of rest. With this, along with a clean diet, and an organized routine; I dont see why you cant do both, if you do in fact love doing both. You will just have to sacrifice a little, to get a little.