I'm sure I remember seeing pink readers at Highbury and Islington (although I can't recall exactly where they are) - would a tap on these suffice rather than going all the way to the gateline and exiting/entering?
They're down on the platforms, by the staircases to the concourse.
You know, it's an interesting thought; I kind of wish someone would have the guts to try it! It is probably true that it wouldn't prevent the charging of two incomplete journeys. However, my thinking is that in an ideal world, one could then phone Oyster customer services and request a refund for the overpayment, such that the final amount charged is merely that of one journey. The passenger would, after all, have complied with all instructions given for how to use an Oyster card, and there's nothing I've seen from TfL that explicitly says making a round trip to the starting station is against the T&Cs or anything.
Like I say, that's in an ideal world. I suppose that if nothing else, I'd be interested to know what an Oyster CSC agent would say in response to an attempt to get a refund in such a situation e.g. what excuse they might use to decline such a refund.