All you have to do is type in the emote while you're casting mount.
Fx:
1. Have /dance in chat
2. press mount key
3. hit enter while casting
4. boom you're now dacing on your hoverboard
(Works with every emote)
You can still move around on your mount and as soon as you stop moving you will start doing the emote again.
This only works with hoverboards as far as I know.
//LAPD