The majority of platforms are 8 car only along the WAML, only a few places are actually 12. Like I said, it will have to be ASDO everywhere thats not 12 car long, which is fine, but there are some issues at some locations such as Enfield Lock and Brimsdown where a platform starter signal will have to be moved in order to allow a 10/12 car train stopped there not to be foul of the crossing.
There's also the discussed problems at Hertford East where both platforms are only 8 car, and apparently there's a lot of work to remove the listed signal box, move the junction and then extend the platforms. Until then its fairly impossible for anything longer than 8 coaches to venture down the branch, which precludes most Lea Valley stations as well (As the Hertford trains are all stations down the Valley.
I'd hazard a guess that the Bishop's Stortford - Stratford services would be the first WA services to be 720 operated, as they seem the most "self contained", and they are the first services to be chopped in the event of disruption/failure.
Currently I believe the only WAML 12 cars are a couple of Kings Lynn/Cambridge peak trains, with extremely limited stops. They used to run some of the STANEX as 12 but not since the last timetable change, and again, they only stopped at 12 car platforms (BIS,HWN,SST,SAW,CHN,TOM).