Personally I never heard of any M&M public available beta.
Which makes me wonder if any of NWC's former employees happen to have one. There's a situation like this in Thief, a game made about the same time as MM6. There are some old beta clips lying around and one person managed to obtain the source code for Thief 1 + 2 (even System Shock 2) and send it to Eidos.
I'm hoping that because MM6 has long since been released, someone may have a beta and is willing to release it. Though this is the ideal situation and chances are, the betas might be erased (though interestingly, I read on a Thief forum that any software that is designed in a game studio does get stored somewhere on the internet or at least some kind of network, though I'll have to see if I can dig up that post so you can understand it better).