Possibly useful: https://github.com/openaps/dexcom_reader. That's the keys to the kingdom for reading data off a Dexcom receiver over the USB port. Certainly the easiest implementation since we have that spare receiver.
But the USB route might pose an expensive problem. I've been told that the Dexcom transmitter somehow increases its signal output strength in response to how far away the receiver is. Not sure how this is accomplished technically - seems like the receiver would have to send back an RSSI or something - but we have seen evidence of this. G4 transmitters are only warranted to last six months, but we routinely get a year out of them because the receiver is always on my daughter's person. For this last transmitter, we tried being super diligent about distance, keeping the receiver in bed with her at night rather than across the room, where we could easily pop in and check it. This transmitter lasted 14 months, a record for us.
Transmitter life is no joke - these things cost $600 a piece, and our insurance doesn't cover them. So using the spare Dexcom receiver might cause a problem, since the transmitter will think the bedside receiver is out of range most of the time. For that matter, I don't know if I can assign one transmitter to two receivers - each transmitter has a unique serial number that you have to enter into the receiver. Keeps your data private if you happen to be near another Dexcom user.
Seems like sniffing the RF might obviate those problems. I don't care about dealing with multiple transmitters, and if there's something about the conversation that controls transmitter strength (and therefore battery life), I can probably control that in software. So I need to learn more about the packet structure and maybe start playing with an RTL-SDR to see what's going on between the transmitter and receiver.