Yeh it's a bit tricky to add new mounts because of the crappy way the animations are handled in Iris. I messed about making a Gerry mount one time which wasn't too bad since it's a similar model to the Little Tiger mount but it still didn't look great because there are several sets of animations for each character when mounted (the bouncing up and down as your riding along, or jumping etc) and they have to match up with the animation of the mount.
Adding a board wouldn't be too hard since I could probably use the character animations from when your riding one of those mantaray mounts, so maybe one day