I've got a 70-300L with a Kenko 1.4x TC on it. IQ is surprisingly good (or maybe those who say "teleconverters make images mush" have a lot higher standards than me), but the AF is practically non-existant on my 7D. It tries to AF, because the camera thinks that it's only f/5.6, but doesn't succeed very well because the aperture is too narrow.
Still, for $100 it doesn't make a bad Manual Focus 420mm f/8 IS lens.
Mirror lenses are hit and miss, some are razor sharp, some may as well be made from old coke bottles. You can get a lot in the ranges of 400-600mm, to 1000mm
, even 2000mm
. That needs a truck-mount more than a 1200 f/5.6L though.
Other Manual Focus options are Pentacon 500mm f/5.6, made for Medium Format and built like a tank (looks not unlike the vivitar 800 above), and Takumar 500mm f/4.5. No idea how well they perform, but they're both good brands who make other razor-sharp lenses, so they shouldn't be too bad...
And FD/FL Manual Focus lenses with EdMika adapters, I haven't seen real reviews of them, but seeing as they're canon brand they shouldn't be too bad...