First, let me say that I am in agreement with your earlier assessment of the UPnP implementation in MythTV... it has been my experience, as well, that most applications supporting UPnP seem to treat it as any other media server, so I doubt there's any fundamental flaw in the MythTV implementation that would cause problems in the connecting client software.

That said, if you've gotten it to list out your videos and start the streaming process, then you've progressed further along than I did, and you've conquered 99.9% of the UPnP portion of the equation...

Here, I would state that the video bitrate is definitely a problem -- perhaps not the only one, but that resolution and bitrate alone are going to keep that video from playing smoothly on your tablet. (if you were to copy the video directly to your tablet, you'd probably see the same exact behavior trying to play it in mplayer via the command line...).

I would recommend rigging up a script on your mythtv box to automatically transcode your recordings to something closer to an appropriate resolution/bitrate for the tablet. You can set it up as a post-recording task (either before or after comm-flagging) to run automatically from MythTV, or you can manually kick off a transcode script on specific recordings you want to watch via the tablet.

To that end, you can set up something using mencoder (probably already installed on your MythTV system) and work out the appropriate command line parameters, or you can adapt the tablet_encode application (referenced on this board fairly often) to handle the task.