I think there's some confusion here between short platforms and platforms that may foul the gauge of a Mk4.
Loughborough used to be a short platform but is now full length, and Market Harborough will be full length when re-modelling is complete. That only leaves Beeston and Long Eaton on the "core" MML, served only by 222s which have SDO. HSTs have grandfather rights for the short platform at Langley Mill but this route will probably lose its London service when the HSTs are replaced by something that doesn't have to get back to a depot in Leeds.
As to clearances I would guess the problem is more likely to be with curved platforms so Wellingborough and Kettering could be candidates.
Market Harborough is also likely to have any clearance issues resolved, since the platforms will largely be new, and despite it being re-announced as a sparkly new project for the CP6 business plan, work is in full swing.
Whilst the Mk4 idea has merit, I see a few issues that I'm sure people in the right places are talking about all this:
1- It's another mid-life re-engineering project on BR-era rolling stock. Similar projects that spring to mind do not seem to have a great track record of on-time, on-budget delivery (230, 769, 321 Renatus, XC/GWR HST mods etc...). OK, this looks less complex, but if all these projects have unforeseen problems that impact delivery, I'd expect this one to be no different. So to plan for this, I guess you need a test set out and about in Q1 2019 to be debugged, and so VTEC needs to release a set of Mk4 probably no later than the end of Q3 2018. Also need EMT to cough up a couple of power cars at a moment when they've just taken additional sets because they didn't have enough to run the 2018-2020 timetable.
2- You're adding another point of failure to a set of trains that are going to be rather elderly by the mid-2020s, and may be getting to the wrong end of the bathtub curve of reliability. Or when you start doing these upgrades you find other stuff that needs fixing (a reason why point 1 can be expected to happen)
3- Even though I expect the clearance work talked about above will need to be done eventually, are NR capable of delivering this by 01.01.2020?
This is not costed in Ian Walmsley's £24M figure.
4- This approach locks in extensive LDHS diesel running from Kettering southwards for several more years - 4/6 EMT services assuming EMUs for Corby. Given the current environment, we can't expect Nottingham & Sheffield to be wired this side of 2030, so you're kicking a lot of the benefits of the current schemes into the long grass. Apply a discount ratio to those benefits... it becomes yet another electrification project that has failed to deliver against its business case.
I guess I'm saying it doesn't feel like a good option, even though it might in fact be the least-worst one.