Quick followup: If E- is not yet full, is there some way you can "decline" to be assigned a seat when you buy your ticket, thereby forcing airport seat assignment? (The idea would be that if you expect a flight to become full in E- but it's not yet when you buy your ticket, then if you can "force" airport seat assignment when you buy your ticket, you could at least allow the possibility that you'd end up with a seat in E+)