I think I have found the solution:
you install flex as a client on a LG/Samsung Connected TV. Supported devices are listed here: http://www.plexapp.com/connected/index.php
It looks like something is also available for Sony TV which includes google sw: http://gigaom.com/video/plex-sony-internet-tv-app/
and then you install Plex server on the QUAD through this extension: enigma2-plugin-extensions-dreamplex
and you should be all set. The issue is that I don't have such TV yet but I ll try with another client like a PC or a tablet/smartphone.
I didn't know Plex before but it looks more simple and better than XBMC for an easy experience.