steve525 broke the news that there’s a Media MVP client for SageTV thanks to an intrepid sagetv user Matt! More info/download at the SageTV MediaMVP client page (note: this isn’t an official SageTV release but it’s very cool nonetheless!)
Now there are a myriad of options for using a mediamvp as a client of other pvr systems.
from the readme
MVP Client for SageTV is a free plugin for SageTV (www.sage.tv).
It allows a Hauppauge MediaMVP device (www.hauppauge.com) to be used
as frontend for SageTV. Support for features like On-Screen-Display, Audio
and Video is close to that supported by the normal SageTV Client.
Exceptions are mostly due to restrictions imposed by the MediaMVP (see
Known Issues below).
The current version is MVP Client 1.0. Updates will be posted on
www.streams.ch as they become available.
Check out the SageTV forums (http://www.sage.tv/forums) for support and
for announcements about new versions (I hope everything works great as
it is and no updates will be needed ;-).