I am finally back to this problem, have been caught up at work and unable to get at this again. I am familiar with the hotkey functionality and am currently using it for the exit, load, and save states.
I tried your response and still not working (tried multiple indices to check if there was an issue with some combination with and without it being hotkeyed). I plugged in my keyboard and tried setting keys for that to and still nothing. Is there anyone who actually has this working on their retropie project?
Have I missed enabling something other than the rewind_enable = true?