A clone can be sent to the rocket just as any other character. However, the special UI for "player on board" doesn't show up. The player is given a "ground" view of the platform as though they can walk around. This isn't an issue in itself, but it has the unfortunate side effect of making it impossible to get the clone off of the platform. The button for it doesn't appear. I've been able to send the clone as an item (rather than a player) and that works, but the clone is stripped of equipment (guns and armour) when picked up, which makes transporting them between planets quite cumbersome.