Interesting usage. I've yet to find anything on it. Sorry.
Does it have to do with Oxbridge's extensive reading list requirements? If so, I don't see why that usage couldn't be applied to other colleges and universities. Mind you, as a new usage, "reading a subject" seems to be in its infancy. It just hasn't had time or chance enough to circulate. And that it started at Oxbridge says a great deal, too; you know how they like their traditions.
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