When Canon updated the EF 400mm f/4 DO IS with a verison II, most felt that was a good sign they were still developing DO technology and lenses and we could see more added to the EF lineup.
An update to the EF 800mm f/5.6L IS has been mentioned ever since the new EF 600mm f/4L IS II was announced back in 2011, though nothing has come to market as of yet. Such a lens is a low volume proposition, and there are a lot of other areas Canon can make money on lenses.
I've seen talk on forums recently about an 800 DO and whether or not folks would buy one, and at what cost.
We're told after a lengthy conversation that Canon is indeed continuing to develop DO super telephoto lenses and that 800mm is one of the optical designs. We've seen patents in the past showing an optical design for an 800mm f/5.6 DO IS.
I got the feeling that any replacement of the 800mm f/5.6L IS would be DO. What would it cost? I'd say somewhere between $15,000-$20,000USD.
Nothing is imminent, but it was a good conversation.