Well, I was thinking about it a bit and came to a few concepts. I'm not a coder or anything but I hope some of those are possible:

1. Put the player in a minecart entity over the mob's head, trace the minecart control and move the mob accordingly

2. The same with a boat.
   The issue would be making those entities invisible, maybe they can be affected by an invisibility potion somehow?

3. Put the player in a flight mode and trace his movement, then move the mob accordingly
   and send a packet that says you're sitting to others
   But there's a problem with the client's view and flying upwards.

I hope I gave you some ideas to work with and that they're not completely useless :)

Thanks for your input. WASD control has been on the "to do" list for while - just haven't had time to experiment lately. However, it is something that I definitely still want to delve into.