As for potters bar terminators ken bid for that before but was rejected as there is no capacity for them to cross the ladder south of the station.
Why stop at stations on or near the boundary (Z6)? Trains from Moorgate should continue on to WGC (or Hertford North/Stevenage) as they do now.
I can't see why they can't become part of LO (whether directly run, or a joint-operation). On the ECML, they'd run exclusively on the slows and if there are additional lines from Ally Pally south then all the better.
I thought the idea was to extend the zones out one day, as London is growing and WGC, Hertford and areas about the same distance outside of the M25 are increasingly becoming part of Greater London.
In fact, the problem now comes down more to the DfT arguing about what smartcards to use - not wanting to extend Oyster (even though this time last year, they were aiming to do just that).
London Overground would offer a metro-like turn up and go service, which would be slower than some existing trains (by always being all stations) but could still prove very popular for the convenience (and less hardship on the odd occasion where you can't board). They'd run around faster trains.
FCC still manages to squeeze in a 2TPH evening peak service (not sure how many in the morning) that shares the slow lines with the 3TPH slows to WGC. 2-3 stations from the end, it's likely to have caught up and may crawl but when I used to use them regularly, the train has already deposited the bulk of passengers by Potters Bar, so only a small number of people are delayed (and probably not delayed at all if they're going to WGC thanks to the recovery time in the timetable).
Could FCC still run services around 4TPH? Possibly not now, but if there are extra tracks brought into service then all it needs to do is be able to get ahead of the slow train by Alexandra Palace.
The question is whether you could fit LO trains to Herford East at that frequency, and other lines that are only two track.