It took a lot of analyses as well as a bunch of trial and errors, but I got the BoltEV-specific PIDs working in the UltraGauge MX in the way I intended. It’s highly likely that ScanGauge can also be programmed in the same manner.
There are some limitations - UG only allows for a maximum of eight custom PIDs and the automatic power on/off function doesn’t quite work as smoothly on the Bolt. But I think all these issues are manageable and not show-stoppers.
I now need to take the car for a drive to make sure that everything is fine with this device plugged in.
On a side note, only seven out of nearly 80 default OBD-II PIDs that UG supports are available on the Bolt. That’s not too surprising, since much of them are relevant only with an internal combustion engine.