I tend to agree with Greenback about the semi-fast idea. I believe that FGW did operate a two-hourly Della to Exeter StD as an extension of the Bedwyn service, but it didn't stop at Bedwyn (not sure if it stopped at Frome either, certainly not Bruton). The biggest problem would probably be pathing, although having the unit sit around in the platforms at Westbury or Frome might help there. The other thing is the fact that FGW doesn't have a high-speed DMU any more, so the service would have to be either an HST or a 158, unless they retain a few Turbos.
It seems a bit unlikely that it will ever happen.