The trouble with that is...
"Matthew from Werribee says he has HZ12345 sitting under his bench"
How many people do you reckon would try to claim it? How would you prove they owned (or still owned) the car it belonged to?
The person seeking the car / engine / whatever has nothing to lose posting the numbers, but I personally wouldn't like to list every engine number I own on a public forum, even the non-original engines in currently registered cars, just on the remote chance someone will see their OEM gear.
Besides that, 95% of cars that gave their engines over the years would be gone forever by now.