If the baseband and associate chipsets are isolated on the other side of a USB connection, why not use (any old dongle) and isolate it with some kind of USB "co

If the baseband and associate chipsets are isolated on the other side of a USB connection, why not use (any old dongle) and isolate it with some kind of USB "condom" with configurable "firewalling" ? In fact, I believe I have seen such devices for USB development - a configurable passthrough USB interface that would allow you to control what gets sent over the interface. That kind of a setup would be much, much easier to develop and safeguard than developing a modem/baseband from the ground up ...