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.
https://www.raspberrypi.org/forums/viewtopic.php?f=38&t=188723&start=1594
Thanks much for the input @zerojay , it got me pointed in the right direction!