Horsey! could be used to do something similar to this. Instead of left-clicking to open the Horsey window, you could right-click and open the stable and choose from there. Of course, that's essentially just using plugin as a quickslot bar, and there's far better plugins for that. Personally, I'd suggest MrJackDaw's
Pop-Up Quickslots. They hide when not in use, and can be used for lots of other things too.
However, there is a MountSelector plugin contained in a collection of beta-era plugins
found here. Don't know if it will do what you want, but it's worth a try.
rushl