UPDATE:

Turns out this is pretty easy. You gently squeeze the plastic retainers
that hold the camera's back from opening completely. Most Fuji cameras
of this era had a "drop-in loading" setup that didn't require opening
the back all the way. When you squeeze the retainers, the camera back
swings open, and there's the battery, plain as day, and looking simple
enough to unscrew from the interior of the camera back door.