The whole station is a fare control area. The only entrance is owned and
managed by London Underground with a tube style gateline. There is a
second gateline between the Jubilee line and the rest of the station so
anyone starting their journey at Stratford or interchanging from the
Jubilee line will be forced to pass a barrier.

Validators on the island platforms are provided for transfer between
'one' services to/from the east and Central/DLR.

Is the North London Line covered by Oyster (other than the bits that are
duplicated by the District/Jubilee)? It's entirely possible to get between
the NLL and Central without going through a barrier.

It's not the easiest thing to remember - IME, it's easy to forget to
touch in/out on the DLR if you only use it occasionally. Much better
when gates force you to touch in/out at system boundaries.

Quite apart from the problem of transfer to the North London/Jubilee Line (I
still can't make out if Oyster charges you for interchanging through those
barriers - I use a travelcard), I just can't imagine trying to add gates
along what are already very narrow island platforms!