Got it all sorted out.
Fruitbox running smoothly on a Jessie-based RetroPie image, straight from the EmulationStation main menu.
And I wrote a how-to, so you can too.
Thanks much for the input @zerojay , it got me pointed in the right direction!
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first