Thanks, Kavalliero. I hope by sharing my experience I can save someone else the frustration.
I was ready to pay for a new one, but my research led me to believe that it may have been discontinued by Ryobi. Part number HM-996515001. If anyone has a lead on available stock, please let me know.
View attachment 110972
My solution is to tie the blue wire to the red battery+ wire through an additional 10 kohm resistor to keep the current around 1.5 mA. I installed the modified charger relay, and... the main relay clicks on when the key is turned, and the mower works again!
If anyone else has fried their charging port, this could be a way around the problem.
Enjoy!