There is client check that prevents casting mounts, but it doesnt check spells cast from Equip effects.
Make a custom item like this shirt with a equip with spell ID of your mount:
Code: http://paste2.org/p/393173
Preview of shirt : http://tinyurl.com/mt7s4p
Now, this will work as long you are in GM mode, because otherwise mangos limitations will not allow you to cast the spell if you are not a GM. >This patch< remove Outland limitation (Making it available to non-GMs players) on spells and works with latest. You still need Cold Weather Flying and the appropriate riding skill levels to ride mounts.