It has occurred to me that one might use the manifold pressure switch (overboost sensor) to switch a small piezo siren (e.g., one of the Mallory Sonalert units) located in the cabin. These can be had surplus for cheap, and many models are loud enough that they would be hard to ignore.

That might be just the ticket for someone who is (1) really cheap and/or (2) philosophically opposed to the stock overboost protection plumbing arrangements.