Great example.
This became known as the “Baralong Incident”.
In a total of 150 engagements, British Q-ships destroyed 14 U-boats and damaged 60, at a cost of 27 Q-ships lost out of 200.
Q-ships were responsible for about 10% of all U-boats sunk, ranking them well below the use of ordinary minefields in effectiveness.
However, I have no problem using this tactic against pirates, but no pussified nation in the Western World would do it.
They do not have the balls of T. Roosevelt, let alone do what is needed to save us.
You're wrong. It's not a matter of "balls." You will note that several countries, the US included, have had the balls to deploy their navies in the region. The problem is that they can't protect all of the thousands of ships over that very large area.
Any country that is willing to put a cruiser in the region, would be willing to put in a decoy. It's probably more a matter of inertia and logistics: you've got to build, deploy, and maintain those decoys.
And.... maybe nobody with the authority to do something like this, has heard about or thought of it.