I'd like to see a new JS event added, for when a player doubleclicks a character. Adding a check for this event near the top of void handleCharDoubleClick() in cPlayerAction.cpp would give scripters the ability to override the mount/dismount-code, and would allow for custom paperdolls for anything from normal player characters to all kinds of NPCs.
The event could be named onCharDoubleClick, and would override the hardcoded stuff completely if a return false is provided in the script. With a return true, both script and code would run.