When trying to play recorded shows through Myth's UPnP server, Samsung TVs complain that they can't recognize the content. This issue is known and fixed upstream (r22356). http://svn.mythtv.org/trac/ticket/7296 Attaching a patch that applies cleanly to mythtv-0.21_p20877. It is not clear to me if 0.21 will get this fix officially or not.
Created attachment 221577 [details, diff] Modify upnp header for Samsung TVs. See upstream bug: http://svn.mythtv.org/trac/ticket/7296
One more note, this patch was taken from my overlay. Just add an appropriate 'epatch' line into the ebuild.
Fix is in 0.22 and 0.21 is unsupported.
I've also confirmed this works just fine for me on 0.22 and my Samsung TV.