Redbooda has a decent team so far and it's not like he isn't aware that the draft is going on. I think you just got to let it be at least until his pick comes up again in the 5th Round.
One idea would be that if a player already has one back pick and it's their turn again, then the Commish has to select the best player available (taking into account position needs) for their original back pick. When said straggler shows up, they can immediately make their current back pick (like we do now) but they can't exchange the player the Commish chose for the earliest back pick, they are stuck with him.
Also, any straggler who throws a tantrum and refuses to play, is penalized by not being allowed to play in the same theme league for two additional seasons. (the Commish can take over that season's team or put it up to a new owner or a current owner willing to enter it. I volunteer if nobody else does)
(Wouldn't it be funny if one of the stragglers won the championship?)