It is registered as a glider, it appears (N328KF),

Heh, heh. Indeed -- "engine type: none". That's rich.

but I couldn't find a Mike Melville with a glider rating in the pilot
certificate database

That's because it's spelled "Melvill", no "e" unlike the author of "Moby
Dick". Getting wrong the name of a world hero (of S. African origin)!? His
FAA listing duly shows "glider". Don't you recall that Mike did test
flights in the 1980s on the "Solitaire" which won the SSA's motorglider
design contest?