Just to make sure you don't get a useful answer, I'll throw in my two cents worth and say that I don't think microwaving would be a problem. If I understand your situation correctly, it sounds like you have a low volume need for the occasional BBQ sandwich in a pub. I don't think you'll have a problem MW the meat, and I think a change in flavor, if any, will be due to the way the meat was held prior to MW than due to the MW itself.
I did a fundraiser today where we sold hotdogs, burgers, chic sandwiches, nachos, and BBQ sandwiches. The BBQ had been frozen in quart containers for about a month with some sauce, and the containers were reheated in the MW (12 minutes at 50% power to thaw, then 4 minutes at 100%) with the BBQ held in a 1/4 size steam table pan until served. I won't tell you the steam table did the meat any good, but the BBQ was fine IMO when it came out of the MW.
From a scientific standpoint, I can see how a MW might change the texture of some foods, but I can't understand the mechanism for the MW to change the flavor unless it has to do with transferring an off flavor from the reheating vessel. So don't reheat in styrofoam?