Dunno if I made it clear, but run a component video out from the AVR to the plasma and hdmi from mede8er to AVR. Surely the tv's got component in? Its just my reasoning though, but a reciever with hdmi upscaling/upconverting capability should be able to do it over component. Even if the display is not full hd it should still be able to display a signal from hd source.
Just try and find a menu on the AVR with regards to video out, select upscale/convert to component and make sure your display has the correct settings for component.
If I'm wrong here, someone please correct me, but this worked for me before.
Also, what happens if you connect the mede8er directly via component to the plasma?