I had never thought of this - what a great idea.
In cases like Avants with tubes dead (and no more replacements available) any replacement panel would be marvelous item. I wouldn't think of scrapping the tube out of a working set but with sets with bad tubes (like a Beovision MX3000s, tubes are told to loose their emission very quickly in these sets, mine is working fine, though, perhaps because I never use it!). Some sort of converter would be needed for the electronics and one problem could be fitting the flat LCD/LED-panel to the curved case/frame of the CRT.
I would believe that the electronics in TV sets last (or can be repaired) much more easily than the tubes, they are big, heavy, sealed, made of glass, contain a vacuum, work on high voltages.... none a reason that makes repairs easy! The electronics would need to be changed/heavily modified in order to make the set work with higher definitions than original but as a way to keep the set working this would be great.
I wouldn't imagine B&O making these (rather some small company specialized to these) but who knows - perhaps one day. Interesting idea at least.
Bang & Olufsen - The art of controlling sound, picture and light