Is it a BMW dealer? or just a used car dealer? If it's a BMW dealer, and the engine was replaced under warranty, they should be able to trace records on their system.
What did you mean by "the dealer doesn't know a thing..." was it a sales rep, or someone in service? Assuming you were at a BMW dealership, and they don't know a thing about the Nikasil issue, they are obviously trying to avoid the question.