Knowing the configuration of the station, this has almost certainly been nixed by the Health & Safety Mafia, on the grounds that the distance between the top of the escalator and where they imagine any queue might be is inadequate.

Besides which, it's a very profitable arrangement for London Transport. If you don't manage to touch in, but then change onto the Underground at Canning Town (like our OP Swiss Tony did), and then have to touch out in Central London, you automatically get fined what, £5 now, on your Oyster card, for an "incomplete journey". Plus London Transport can use all the "incomplete journeys" detected to pretend they were all fare fraudsters that their skill has detected, which makes for such an exciting report in the news every year.