How exactly do you propose that they set up a radio to mimic human brain waves? Not only would this be extremely technically complicated, almost certainly beyond Harry's resources or knowhow, even if the source of magic actually constantly reads wizards' brain waves, what reason do we have to suspect that it would not be able to distinguish neurological activity from a radio transmitter projecting the same patterns?

It sounds like a huge amount of work for a test which stands a great chance of being completely useless given that Harry doesn't have anywhere near enough data to home in on this as a meaningful avenue for investigation.