Don't ya hate it when you have a problem and you go the support communities and all you find are posts saying they have the same problem. I thought that was what the "I have the same problem" link was for. Misery breeds company I guess but it clogs the posts.
So it's really refreshing having contributiors add value by telling us what they found out about the issue
and especially to peterfromsarasota for a work-around.
I tried this by using my iMac as the second monitor and it worked although the attendees/viewers had to deal with a massive screen resolution. So I fiddled around with the 2nd screen's resolution settings during a GTM and managed to get the"Grey Message of Despair" (ie. the rMBP re-started) but eventually found if I set the second display resolution up first, then started the meeting and shared this screen all worked well and the attendees get a reasonable vision.
Of course it won't work stand-alone when on the road but half a loaf is better than none.
Thanks for the workaround peteratsarasota and to others on the post especially Orion the originator