I use a "3 in 1" button macro.
/cast [harm] Sinister Strike
/startattack
/dismount
/use [noharm] Reins of the Cobalt War Talbuk
If targeting nothing, or targeting a friendly, you will mount up. If you are on a mount, you will be dismounted. However, if targeting an enemy, you will start auto attacking as well as Sinister Striking (spamming the key won't toggle auto attack, no damage loss). This can be replaced with Backstab, Bloodthirst or whatever you primarily attack with.
I have my flying mount on the next key over, which is also my primary finisher (SnD) and can be anything as well.
/cast [harm] Slice and Dice
/dismount
/use [noharm] Swift Green Gryphon
A bit simpler since there is no auto attack start.
Change the ability/mount and you've got what you were asking for plus a mount key.
