With this hyd failure (767) ETOPS is not affected (in my experience).
Well, you do lose the HDG (aka HMG in the MEL). And, as you point out, you can be dispatched ER(=ETOPS) with the HDG inop with the other generators operating normally. For over 120 minutes ETOPS you need to run the APU during the ETOPS part of the flight.
Whether it is wise or legal to continue ETOPS with the loss of center hydraulics will be debated by the geniuses with diagrams and highlighters in the training building for years to come.
Some news reports seem to indicate that the hydraulic problem was not evident until the gear was lowered for landing. Anybody have an update on that?