Well if the model was solid on all surfaces it might be possible. Lets say the ship is moving forward, and you walk backward. Then the it would add the movements from the ship, or subtract depending on your direction of travel on the ship, to your movement speed. So you could effectivly walk on the ship. Then when you stop moving for a duration of time a mount could be spawned, if possible, and you'd mount to it were you were.
If the ship was going 50 then:
Toward the front of the ship would be normal movement plus 50.
Toward the back would be normal minus 50.
Toward the sides would be normal forward speed plus 50 sidespeed, and it would have to atomaticly walk sideways for the player toward the front.
The problem there is the sideways animation would make it look funny to walk sideways. You'd have to find a ratio for the angle of the player, and the speeds that would be needed, and you'd need to have the speed scaled to the ships current speed.