I got the feeling, I saw the Nalak mount already 10+ times, but I dont notice it. Looks way to much like the Celestials Rep Mount.
Also I got the same luck as OP, I even bought some Battlechests & used the free MoP Trial to farm the mount, and nothing. Got in 1 Week about 100 kills on each boss (create four Lvl One characters + Main character & summoned them to the boss and delete them after Nalak).
I think I got around 500Kills on Sha/Gall & around 400 on Nalak/Oon. On those kills I only saw twice the Oondasta Mount for other players, but no other mount.