As one priority of the No 4/5 duties (in big games) in our area is to 'have 15 on 15 where possible' in referring to subs movement and getting YC players on, we would often allow the subs to occur when conversions were being taken (and radio them into ref). So I don't see any reason that if YC 10 mins expired and he can come back on to then allow them to kick (within the 90sec of course). Advantage of this also is the ref can 'chat' to him after the kick about maintaining his discipline for the rest of the game. Win/win IMO. But certainly would like to know if there is a law that prevents it?
Edit. After a quick look. Think I might have to retract that statement! (My highlights)
Ie
[LAWS]Dead: The ball is out of play. This happens when the ball has gone outside the playing area and remained there, or when the referee has blown the whistle to indicate a stoppage in play, or when a conversion kick has been taken.[/LAWS]
And
[LAWS]3.11 Player wishing to rejoin the match
(b)
A player who leaves a match because of injury or any other reason must not rejoin the match until the referee permits the player to return. The referee must not let a player rejoin a match until the ball is dead.[/LAWS]
So it seems the ball is not dead until AFTER the conversion is taken, unless you argue that blowing the whistle for the try is also to indicate a stoppage?
After thinking on this, I would not allow the YC kicker returning to take the kick (I would allow him on in prep for restart), but should, say the the opp kick the ball away and I blow time off, then I'd be inclined to then let the YC kicker to take it (ie ball is now dead by fault of the opp, and so kicker can now enter the play when I restart time for the conversion kick). Though I sense I'm setting myself up here if I did allow it.