Leeds-Lancaster/Morecambe/Ribblehead and Metrocentre-Morpeth/Chathill could do with a couple.
Metrocentre-Morpeth/Chathill seems ideal for these trains.
The unelectrified sections are fairly short, which means that the technology would be first tested on a fairly forgiving line.
Much as I appreciate the enthusiasm for Barrow, that'd mean being "off wires" for a couple of hours at a time (given how slow the line is beyond Lancaster).
The Morpeth duties can't currently be pure-EMU operated (since BR didn't bother wiring the headshunt at Morpeth - and there's no realistic hope of setting up a team to do this frustratingly short section), but an EMU doing the current Newcastle - Metro Centre - Newcastle - Morpeth - Newcastle - Metro Centre - Newcastle diagram would spend 95% of it's time under the wires, so no great drain on engines for long periods.
Morpeth would be just big enough to suit a trial and could be split out of any interworking with other services (e.g. Carlisle duties), rather than the complexities of splitting up diagrams around Manchester/ Leeds.
It's fairly flat, it would mean keeping the trains relatively close to the depot (Morpeth - Heaton is only fifteen miles if something goes wrong, Metro Centre - Heaton is about five miles, so you'd not be dragging something back from rural Cumbria if something went belly-up).
It may also be good PR to give some new technology to the north east, given that they aren't getting 195s/ 331s under the new Northern franchise (the only "new" trains are going to be refurbished 158s).
Plus, upgrading the Morpeth trains from 75mph DMUs (142s/ 156s) to 100mph EMUs might be rather handy in terms of getting out of the way of VTEC/ XC services through Cramlington.
(I like the Airedale idea of using these trains to permit a peak-time Settle service as an extension of an existing Leeds - Skipton diagram, but I think that the first place they are tested should be relatively simple/ small-scale/ "quick win" - lets get the technology working before the crayons come out)