You could also modify your Warcraft macro to something like:

/cast [nomounted] Name of Your Mount.

That way, you can spam the key as much as you want, and toons who are already mounted will not dismount. A graphical notification would be nice.

Not sure if you can do this with Power Aura's or not.

You could probably set up MSBT (Mik's Scrolling Battle Text) to display when a toon gains a buff, specifically the icon from being mounted with a given mount.