mounting of this nature would require a whole lot of work if it were to even be considered or implemented at all. there would need to be sprites for each of the mounting positions for every mount available, not to mention source code modification to allow for such a thing.
it's an interesting idea, but i highly doubt it would be implemented any time soon, if at all. there are quite a number of other "features" and necessities that the server needs sooner, like being up-to-date with all the episodes and other features that gravity has released quite some time ago.
edit: oh, forgot to mention, there are some rather interesting side features that are being worked on and hopefully soon to be implemented, just read the topic
here to see some of these ideas. i am sure at least one of the ideas listed there will wet your appetite =)