Have you managed to build the MythTV devices? I imagine this is where the focus of your fixes would be.
I haven't actually tried this yet. I've build the Orbiter and some other modules. For MythTV the first thing I want to do is actually just configuration stuff. When running full screen it shouldn't be using a window frame, when recording NTSC it should record the captions, when recording PAL it should record teletext, it should be compiled with threading support, it should use a decent V-Sync (RTC or OpenGL V-Sync), the video directory should use the XFS filesystem and a daily defrag should be run, autodelete should leave more of the disk available for non-mythtv stuff by default to avoid both fragmentation and because we should assume that the partition contains more than MythTV recordings, we should set up the Linux kernel for A/V (enabling preempt smaller time slices), MythTV should be coordinating the time of the Schedules Direct download, MythTV should be themed to match the LinuxMCE theme, the orbiter OSD and MythTV OSD's are not coordinated, etc. etc.
MythTV is also just not as stable as it should be in the LinuxMCE environment. This will probably take a bit more work to get to the bottom of and fix. It looks like we're pulling some qt 3.3.8 packages, which is known not to work well in multi-threaded environments, but the problem is probably more complicated than that.
My own pet peeve is the inability to configure LinuxMCE to use an existing MythTV frontend - I have a frontend that has been recording since September 2004. It's rock solid and my family relies on it heavily. I am not willing to replace that with LinuxMCE, at least not in the foreseeable future.
I think you must mean an existing backend. This can be done, but is a bit problematic at the moment because of how the listings are downloaded. You basically need to make the LinuxMCE core machine the master backend, and turn your existing master backend into a slave backend. Or you can just keep both master backend running independently and point your frontend to the one you want to watch recordings from. This can be done by making a copy of your ~/.mythtv directory and editing the mysql.txt in that new directory with the DB info for the other master backend; then just run mythfrontend from that directory when you want to connect to the other master backend.
I personally think that there is a lot of issues in how tightly coupled the orbiter has become to some of the media devices and think that this is the cause of a lot of issues, as it makes the code brittle and too hard to do simple changes.
I also think that all teh Media player (XXX_player) devices should be contained by the media director itself and not the onscreen_orbiter - but that is another issue.
Can you give an example?