It's worth bearing in mind FR and MOL's reputation for blurting out anything that generates publicity. I do believe the phrase "free bl*w j*bs in business class" was used by MOL in connection with this alleged trans-atlantic idea. Maybe one to file alongside the "£1 for the loo on flights of less than one hour" concept as well.
But let's just assume, for one horrifying moment, that FR (or some other organisation) decided to go transatlantic on the true loco model.
1) They'd be hard pressed to get below CO's entry fares on BRS-EWR.
2) The extras would be doubtless be ridiculous.
3) A large number of people who will bear FR for up to 2 hours would baulk at that for 7 hours, particularly on the overnight coming back the other way.
4) They'd clearly have to fly to the middle of nowhere, which in the US, can be considerably middle of nowhere. Getting slots at EWR/LGA/JFK/IAD/BOS/ORD is inconceivable on a low-cost model.
5) There doesn't seem to be the latent demand in the West Country for a US route. If a loco managed to generate such demand (£99??), then it would be pulling people in from all over the place, which doesn't seem to be very compatible with the airport's position of being for the West Country and contributing to our economy.
6) If you were going to do it from BRS, why they heck wouldn't you start at STN or LTN first?
Just some thoughts.
I think we might have to conclude that our region doesn't (yet) have the demand to warrant a direct US link. As others have said, if CO can't make it work as a scheduled, it seems unlikely anyone else would. (Bearing in mind CO seem happy with BFS, BHX, EDI, GLA). I think we just have to (glumly) accept that too many people in the West go to Heathrow rather than to Lulsgate.