Well, I feel a bit silly now. Back to the drawing board...

It seems that MS Pocket Streets will allow you to change the GPS "COM" port input to *anything* once it's begun to accept valid position data from a functioning GPS. After changing the COM input, you can even return to the GPS Info screen and watch its data fields blink as if they were really being updated! But they apparently are NOT updating.

Does anyone know a way to make this work?