They wouldn't want to use 75mph units all the way from St Pancras, for a start it would cause big pathing problems south of Bedford, and seriously increased journey time. Also, I'm sure the Skeg train will run in an existing Nottingham path. I can't see them hiring in 170s for the service, and refreshing drivers on them, and even 158s wouldn't be good on the MML, plus with any sence they will be putting extra capacity on Liverpool to Norwich aswell summer saturdays, so using most 158s. If this does happen, HSTs do seem the best option.
As for the loco ban, I thought that had been lifted now, and locos had been allowed to run. Also, a HST is only 70t, so a lot lighter than most locos (often well over 100t), and have low unsprung weight. It's about time NR delt with the track aswell since if it's not in a state to carry locos it must be getting towards a state where it can't carry units either, not with a comfortable and safe ride.
I can't see them upgrading the line just for this though, since it's only going to be on train a day on saturdays over the summer, probably 2 or 3 months at most.