Over the next few weeks you may well see the odd 317 or even 321 on a Shenfield metro service while Ilford Depot make sure the 315s are handed over to TfL is as reasonable nick as possible for a 35 year old train. Even so AGA will continue to maintain them on behalf of MTR and LOROL.
Once the big split occurs on 31 May 2015, only the AGA drivers at Ilford will sign 317s, MTR Ilford drivers will only work 315s, so while you can't say it won't happen, the chance of a 317 on the GEML is rather limited, and not planned.
The 12 x 317/7 on the 17.02 LST - NRW lasted all of 8 weeks, and if reintroduced now would see many passengers standing, not a good idea.
When 317s were used on boat trains and some Ipswich off peak services they were on hire to Anglia Railways, not FGE, all a long time ago.
And finally Clacton Depot are doing some refurb work on 317s, so you will see the odd ECS down there.
Think that tidys up a few loose ends