The capacity is definitely needed, but it would require a lot of jigging around of stops and crew. The Mk4 couldn’t do todays 1V90, for example, as it can’t call at the request stops on the island, the locals between Bangor & Llan Junction or Shotton. It would also require to only be on trains on the coast worked purely by Holyhead crews as Llandudno Jn & Chester don’t sign them, and to use 1V90 as an example it’s also a Shrewsbury driver between Chester & Shrewsbury who don’t touch them either.
Definitely beneficial to get it out over the summer weekends and whilst it’s probably not impossible, it probably is the resourcing that causes the most issues and headaches, as well as the “missed” stops.
It would certainly be good to get it out on Holyhead - Crewe runs if it was possible, like the 67/Mk3 workings did a few summers ago, which I think doubled up as a maintenance swap between Crewe & Holyhead over the weekend.