My thoughts exactly. I've never known a journal to do anything to promote the fact that someone has reviewed for them (eg produce a letter on request that states which articles someone has reviewed for them). It'd be a nice and free way of creating value to reviewers who generally work for free, but I don't know any journal who's ever done this.
Of the people I meet at a conference, how do I figure out who has been a reviewer for Nature, and how do I verify that claim?