I have used a Z41 with McClicky installed (softpress button) with my MD2, MD3 and MD4 and have had no ill effects. The threads seem to match very closely and I haven't had any problems so far. I'm not sure if the twisty action would be as smooth as on a surefire body, and I have found the anno wears differently, but apart from that it should be fine. There shouldn't be an issue with current unless you are using something significantly higher powered (like putting a tripple in your MD2 head)
I have also used an A19 extender on an MD2 and it worked fine as well.