If you install the Hyper-V role on a Windows 2008 R2 Server running Active Directory (on the host), will Microsoft provide support? Is this MS approved or will they say this configuration is not supported and hang up?
I don't think it's unsupported as much as it is unrecommended. It is a best practice for Hyper V to be installed on a server with no other roles enabled. See the following link:
You might also want to check this out:
Works, is supported. If it would not be supported MS would have to block it or provide a warning when you install both roles.
it is not recommended because in LARGER installations you WILL possibly run into performance issues.
I have 2 servers like that, works like a charm.