As xNineLives has pointed, I have been able to record 1920x816 (2:39) and over if you choose zoom x5, not more than 8 secs at that res, but for me enough to get a shot. Continuous recording is another matter, which like everyone is saying for our 6D seems pretty difficult to archive.
Without going to crop mode I think we can not go further than 1792x752 (2:39) and I am getting round 11-12 secs, which is not bad either, upscaling that to 1080p you get pretty good results.