When I raided HC with D&T, we "practiced" quite a bit on the PTR to get to know the encounters before they dropped on patch day so we could be literally "World's First at ...." The PTR is inherently unstable. It crashes, it has bugs, etc. Mats for items are very very sparse.
We would get little to no support from Blizzard. Even less than we do now. I know it sounds like an awesome idea, and it is an awesome idea when timed correctly, more on that later. Now, might not be the correct time.
The correct time to try is when the servers come up in preparation for 7.0. BG queueing is not immediately available, so we would have to watch for that. And there is usually more than one server a available. We would need to communicate. We could also duplicate one some our toons over there for free with bags packed full of twink gear. And distribute it amongst each other. But that is many many months away. However, now would be the time to start planning for such an "event". Failing to plan is planning to fail.
/cheers