I imagine that to be the case, the Thames Valley Branches would be ripe contenders for IPEMUs.
Hmm how about the viability for longer distance journeys, maybe an IPEMU pendo conversion for use on the Euston - Chester/Holyhead runs, the majority of the run is under the wires anyway.
I'd be surprised if Crewe - Holyhead could be done entirely on batteries. The range of the class 379 IPEMU experiment was approximately 60 miles, and required 2 hours of charging for 1 hour of operation. Either a serious improvement is going to be needed in battery capacity, or else some intermediate AC electrification would be needed for charging en-route.
Even over the NDL the expermental IPEMU in its current form would struggle to be a replacement for the existing DMUs. There's two sections of non-electrified route, 12 miles from Aldershot South Junction to Wokingham, and 17 miles from Shalford Junction to Reigate, so well below the 60 mile range of the experimental IPEMU for a single leg. However, the time spent by a current stopping service over the non-electrified parts is 52 minutes, so let's say that 1 hour 45 is required for recharging. The time spent by the unit on the electrified sections is only around 36 minutes, including the turn around times at each end. Therefore additional dwell time would have to be added, meaning more units would be needed to replace the DMUs. Extra platforms or sidings are therefore required to make this work, and possibly some extension of the electrification to increase opportunities for charging the batteries.
Edit: the Gatwick services would also run out of power, with only 48 minutes to recharge per leg, so the limiting factor seems to be either the amount of power which can be drawn from the supply, or more likely the charging rate of the batteries. This problem also throws a spanner in the works for the Thames Valley branches, which would be mostly unelectrified. Ironically the Greenford branch has a better chance of working, with the longer run into Paddington, but this won't be the case in the future with West Ealing bay now installed!