I don't hink mounting and dismounting would be possible. They are outside the scope of the game mechanics when it comes to anything other than siege equipment.

You could make the unit move as a 'mount' with a crew on board. The elephant, for example, could probably be replaced by the cart mesh, and the descr_skeleton entries replaced with animations from the cart to make it roll if movement were needed. The 'crew' would be the elephant riders.

The crew would have a nasty tendancy to shoot through the wood of the wall... which is a bit naughty but unavoidable... and the AI behaviour of the unit would be a little 'bizarre' but there is no reason something couldn't be done.