East Coast services? I don't think you could do that, because the Hertford Loop is only two track, and it has a top speed of 75mph.
Well currently there are three trains per hour between Alexandra Palace and Hertford North, with one of these continuing north to Stevenage/Letchworth Garden City.
Now, the timetabled transit is 29 minutes between Palace and HFN, which is a distance of ~14.6 miles, which would be tarnsitted in roughly 12 minutes if the train was to sustain 75mph throughout.
This all works out that there might be two paths an hour free on the Hertford Loop if the InterCity trains transit the route at the linespeed of 75mph without slowing, and the line is resignalled to allow ~2 minute headways (as could happen when ERTMS arrives). These would be catching up to the two trains that terminate at HFN. (So in front of one of the terminators and the train that continues through Stevenage).
This is all very iffy, however if newer stock was to be deployed on the Hertford Loop these paths could be firmed up due to the reduced transit time of the stopping services.
I think the cost is worth it considering the fact that the current 313s are getting on.
But anyway, there may be two slightly iffy paths on the Hertford Loop that allow it to be transited at full line speed and thus only loose a few minutes relative to the regular route.
I would recommend that these paths be used on secondary services such as any future Lincoln trains and those operated by GC or to destinations like Skipton where the convenience of a single seat service is likely more of a factor than shaving a few minutes of headline times.
A Cl313 only has 880hp and one third of its axles powered, you could easily shave five minutes off that journey time with modern stock IMO.