I recently realized something similar. I think that usb-proxy[1] could be used to force what looks like a mass storage device to stay a mass storage device, and likely could be built to be a sort of firewall. usb-proxy is a toy project showing that you can physically "MITM" USB with an RPi, but it could be a starting point.
[1] https://github.com/AristoChen/usb-proxy