I think he wanted to use the Mount ability like the Archer + Hippogriff?
You also need to consider his bit about matching items and XP. I'll edit shortly with the results of a test I'm running.
Edit: Well okay, you definitely cannot use the Mount Hippogriff ability...
I believe your best bet would be to find a hero with the Alternate animations, mounted. (You know like the Demon Hunter's alternate anims are the Demon, and the MK's alternate anims are the pale-white version.) That way you could use the Metamorphosis ability, but just do a check to make sure the "mount" unit is within X range when Unit Begins Casting the ability, and remove the "mount" unit when Unit Beings Effect of ability.
In addition, you could go into the Gameplay Constants and make Metamorphosis de-activateable, and make sure your version has an infinite duration and low cooldown, as well as adding a trigger to newly create the mount when the player deactivates the ability.
Another option for deactivation might be to give the hero a set of Cargo abilities and then move the Mount into the Hero when the ability is used, and then simply check to see when the Mount is ejected, and deactivate the mounted ability when that occurs...I don't know if you can detect when a toggleable ability is turned off.
Hope this helps