I like Arup's research into this... the M4/Wales corridor for services out of Paddington are currently aligned in parallel 1 mile north of the M4 near LHR; the longest bit for HEX to travel (of its 15 minute journey) is the trip from PAD to HAY, west of which it branches off to tunnel through underneath the M4, the M4 spur etc into T1/2/3 (CBS) and then T4/5.
AirTrack suggests a 6 trains an hour connection, allows for 2 trains an hour from Reading, Guildford, and Woking. It's a start, but is that enough? Considering that there are some services into Victoria (to connect to GEX) run every 15 minutes, I don't know if it would be.
But at least the thought is already there... it's just the time frame that stinks (unless that's the same timeframe for 09/27C).
S.