Several comments here:-
Firstly, would it require an additional diagram? If so, can't see it is justified when compared with other uses you might put a set to. (Cue Graham....)
Fairly certain it would - turnarounds at Bristol TM are 15-25 minutes and I can't see a unit doing the trip to Parson Street at least and back within that timeframe including the minimum turnaround time.
Secondly, it's not close enough to either Ashton Court or Long Ashton.
Finally, I'm sure some sort of signalling/track changes would be involved, if it wasn't to completely screw up the freight service, as unless I'm mistaken, once past Ashton Jn a train will completely block the section all the way down to Portbury.
So, nice try, FOSBR but I can't see this one working.
Your thought on the signalling is correct. It's OTW down there.