Ark, I thought about that solution (or using first, second, and third endings) as well as trying to use a DS al Sign. However, you stated that you wanted a true coda (assuming you want to play through the AA'BA'B' structure more than once). In your solution, you are using the coda to create B' and you can't have two codas! All this to avoid repeating a couple of lines of chords. As I said, if the A and B sections are not too long, my solution fits on a single iRP page. You lose nothing by writing out the entire A section and creating A'BA'B' using first and second endings.

The whole thing is a nice riddle. What song are you trying to enter?