Andrew - you have more experience than I in actually attempting this integration so I respect your view on that, however it is an interesting discussion that although getting OT I think is worth exploring more....
Some of my views (could be flawed?);
The system provides three core capabilities that are implemented by MythTV/VDR.
With the main system capabilities being;
* Display the selected video stream
* Controls (Pause, review, etc)
* Change Channel/ Source
* Record current
* Information display (typically from EPG)
Main system capabilities;
* Browse EPG
* Create recording schedule
* Edit recording schedule
Main system capabilities;
* Browse / search recordings
* Display the select video recording
* Controls (play, pause, rewind etc )
* Manage ( keep, delete etc )
Looking at each of these. LiveTV is interesting and I think that this capability should extend to the viewing of any form of video feed. The display of the video stream is the bit where I think we need to be interfacing directly with the MythTV backend. I think this is not beyond the scope of current orbiter. The next three are all capabilities that don't require UI aspects and I believe the Information Display should be possible if the information can be effectively passed as data from the backend. The PVR software in this instance could be one of many linuxMCE plugins that provide these capabilities. Other plugins could also provide the same capabilities from other sources - like Internet streams, WebCams, Captured video feeds from settop boxes etc. Perhaps a separate plugin/player combo is needed to sit on top of these and group together all the channels/sources and manage what plugin is serving up the video at what time.
Scheduling is where perhaps we would struggle with constraints in Orbiter for EPG browsing but from what I gather Thom has made some advancements here using the dynamic datagrids etc. The create/edit recording capabilities might involve some work - but not insurmountable
Recordings should entirely fall into the same mechanism as our current 'Video' capabilities. This might need some additional features/functionality to get it to the state that MythTV is but I don't think that Orbiter would limit this too much (could be wrong)
The capabilities that still present a problem (in my mind) are features such as commercial skipping and the big one (for me) is support in the Recordings area to watch/stream recordings that are still recording. If we are entirely using the 'Video' functionality then media DB would need the incomplete streams added and the player (Xine) would need to be able to play these.
I am keen to explore a few of these ideas whenever time permits. My knowledge is currently deficient in some of the HADDesigner stuff. I have only explored adding/moving a few buttons here and there. I am interested in how datagrids can be used.
Anyway - certainly going OT and is better aligned with a development thread but makes for interesting discussion I think