No, I would argue that only players on the pitch at the point the try is scored can take the conversion (same logic applies to substitutes after all)
Edit: Just realised that substitutes not kicking is an RFU Reg not a law, so that may make my theory hold less water, however I think not allowing them after a YC is within the same spirit
Clearly if it happened in your game today you made a decision.
What law reference have you found to justify (or otherwise) that decision?
In my game I didn't let him take the kick
Reflecting now, I think I was wrong
(But I am comforted by the refs here with the same decision)
Decorily, I think you are saying : let him back on and let him take the kick
From my game today
Red 10 has been in the bin for 09:50s when Red score a try.
Can Red 10 come back on the pitch (after waiting 10s) and take the conversion?
He is in the bin when the try is scored. The Law allows 90 seconds from the scoring of the try to completion of the conversion. The clock is still running and so the ball is (technically) not dead.
He can only come back to the Field once the conversion has been taken.
The clock is normally running when the ball is dead.
For instance at a scrum
Ball is dead
Clock is running
YC players can return and take part in the scrum