Reading some material by naval architect Nils Lucander, he describes a
rudder that adds a "wedge shaped top plate" to the blade. According to
Lucander, this plate "acts as a permanent trim tab while also
directing propeller wash aft instead of letting it run up to the
surface."

Sounds like a pretty good idea at first exposure.

Anybody have an experience on a vessel with a top plate on the rudder
blade?

Sounds like the same sort of efficiecy theory that works with Kortz
nozzles.....