Just to follow up, I received the Balboas yesterday, and they do seem to work - with a few caveats.
The electronics are essentially identical. It is the same processor running at the same voltage and clock rate. All GPIO pin assignments and I2C addresses are the same for all of the integrated sensors.
The Beta 4 WPILibPi image was able to flash firmware onto the Balboa controller, and I was able to drive the platform around using the Romi Reference sample project.
The Balboa doesn’t have the same 3-rail header area for the spare GPIO pins, so wiring up other peripherals will be different. And it is not possible to gear the motors for the same encoder resolution as the Romi, but you can get pretty close. Also, the caster kit doesn’t set the controller parallel to the ground, so there could be some differences in how the IMU behaves on this platform.
Best bet is to get actual Romis to use, or just run WPILibPi on a bare Raspberry Pi with no robot. But if you did need to drive around and you can’t wait a couple weeks for a Romi, the Balboa can be made to work.
EDIT: If you do buy a Balboa, don’t forget to order motors and wheels. (I used the 50:1 motors.) It also does not include the 2x20 female header or standoffs for mounting the Pi.