(Sorry for the long post)
I'm next to positive the ID observation you made is not correct (but I absolutely may be incorrect since I don't have an AV!). I've testing the device id with both the bravias and two blurays and they work fine. They don't really care as long as the content follows "some name:xx-xx-xx-xx-xx-xx". If you are running that register URL with the same deviceid as the remote AND you never cleared out the old registration on the AV - the register appears to work because it's a renewal of an existing device id rather than a true register (again, that's how the bravias/blurays work).
Now the one thing I'm unsure of is the "xx-xx-xx-xx-xx-xx" - that really should be a valid mac address of the device registering - however on your system I can't seem to get the local mac address (weird in itself) - so I hardcoded the dummy 00-11-22-33-44-55. Again shouldn't matter (works on both of my test beds).
Would your mind trying the following tests:
1) Go to the AV itself and delete all media remotes - try the register with the "MediaRemote%3Ab8-27-eb-fb-3f-15" type - success or not?
2) Delete all media remotes - try the register with "MediaRemote%3A00-11-22-33-44-55" - success or not?
3) Delete all media remotes - try the register with "openHAB%3A00-11-22-33-44-55" - success or not?
4) Delete all media remotes - try the register with "openHAB%3Ab8-27-eb-fb-3f-15" - success or not?
That would tell me alot about how the AV responds to the device id format
Note: your point #2 is ABSOLUTELY correct. I forgot that my current process is a two phase registration (request then approve) and I modified the "approve" part of that with the "initial" - forgetting the first part which has the "new". That's what you were encountering in this weekend. Once you run the above tests - I'll make changes based on the results and hopefully have something today that will work.
Thanks for all your help!