You forgot that the importer may not import everything. There might be some sort of script in each skeleton specifying what is used as a mount point. You can't make anything except elephant, horse and camel skeletons to be mounts. I've spent weeks working on that trying all types of skeletons.

We can't even add a new bone to the skeleton. The exporter only export rotations and coordinate of the scene root. We can't even change that fact although there has to be a control where scene root is allowed coordinate change. Until someone or CA releases a tool to really dissect a skeleton and modify EVERYTHING in it, I think we're sh*t out of luck. And I have a feeling that those are hardcoded, since the game engine needs to call up events. Since that is locked, I'm afraid the event triggers in skeleton is locked too.