Hi Russ,
I've discussed this with Lee and Ashley and have found it is all possible. Attached is my question in diagram form (the same as yours). I was hoping to have it installed and documented but delayed at the moment.
The main thing is, yes, what you want can be done......it just hasn't been yet 
Well I wired it all up as per your details and went and tried it ... and nothing.. Dynon doesn't see the traffic from the FX and the FX doesn't see the GPS from the Dynon. I wired up the PAW supplied RJ12 connector cable cut in half to a DB9 serial connector to simply mirror the USB to DB9 serial cable currently running from the Rosetta so it seems some clarification of the connections is required - I'll be on to the office tomorrow.
Cables wired as follows :-
PAW supplied leads
RJ12 -> DB9
6- white - GRND — to pin 5 on DB9
5- black - RS232 data out to pin 3 TX
4- red - RS232 data in to pin 2 RX
3-green
2-yellow
1-blue - 12V out (DO NOT CONNECT)