I can't think of any flying mount you can get from grinding rep in a dungeon. The Netherwing drake is pretty awesome though, and it doesn't take that long to get. At level 70 much more realistic than the FL flying mount, too.

Well ofcourse it's not viable for a level 70 to kill level 85 mobs + elites, they only good mount is the drake for rep at level 70, but why would you dungeon farm to get to 70 anyway, it's so much faster to level up the normal way, but whatever, to each their own.

If you wanted to do 70 dungeons you can get the cenarion war hippogryph from Cenarion Expedition rep, but tbh just level to 85 get some gear then worry about this sort of stuff

mmm, 70 mounts so better than the 85 and 80 mounts