Peter,
In theory this sounds a dead simple solution and maybe at this RS232 level would work wonderfully! My formal life involved controlling robots with RS232 and splitting caused all sorts of issues, however we were using handshaking as well RTS etc.
FLARM somewhere in its documents says that the signal should not be split, but of course that could just mean Air Avionics worried they may not be able to rip off customers. After all it is already split because the Air Connect can send to WiFi and RS232 to MGL as there are two ports!. In my simple mind this is already split!
FWIW, I used to run a PFC with 'AA' splitter to feed Butterfly and a Flarm LED display, but gave up due to problems with the PFC and moved over to PilotAware. I have now 'expanded' to incorporate Flarm again and use a simple splitter to split the data going from my FlarmMouse to my LXNav LED display to also go via an RS232 to USB converter to feed the PAW and it seems to work fine.
I can of course thinking about this logically and having had a couple of glasses of red wine resolve this simply - MAYBE The Butterfly RS232 WiFi is no longer used as the iPAd now is dedicated to the PAW. SO Spare PAW port needs a USB / RS232 converter and Connect FLARM TXD to PAW RCD plus GND.
That's effectively what I'm hoping to do with the PowerFlarm Core. Our CFI wants to be able to run multiple tablet displays when he's off touring with his better half. He has already tried parallelling 2 x 'Butterflies' but that doesn't work, so hopefully replacing the Butterfly with PAW will let him connect up to 4 devices via PAW WiFi and also see P3i aircraft as a bonus.
Only slight (hopefully) issue is the Power FLARM also throws out ADS-B stuff which means I would be duplicating data arriving at the PAW.
I have already discussed this with Lee and we are pretty sure this should not cause a problem, but the proof will of course be in the testing!
Also I would not be getting mode C & S in the MGL as it is only taking data from the POwer FLARM.
So you are presumably using a 'Basic' PowerFlarm rather than PF Core?
So here is another possible.
Power FLARM TXD to Spare PAW port via RS232 RCD then using same RS232 connector, TXD output MGL iEFIS. Theory says the PAW becomes the source of both data sets to WiFi and also RS232 to MGL. Sounds sound, but not sure from a PAW software aspect.
I'm not up to speed with data supply to your MGL and not something I have tried, so can't really advise. I have a suspicion that trying to supply data in and back out via the same port might be an issue. Let me do the PFC tests and I will have a better idea as to what works and what doesn't. Bear with me.
Regards
Peter