Ian not at home right now.
The hotkey functionality is also mentioned at https://github.com/retropie/RetroPie-Setup/wiki/Does-anybody-know-if-there’s-a-way-to-edit-the-retroarch.cfg-to-give-me-the-ability-to-exit-an-emulator-by-using-the-controller%3F
I would guess that for rewind the configuration would look like
input_rewind_btn = 1 (or some other button index)
