CEOs and other businessmen from all over the U.S., Europe and Japan were used as luxury guinea pigs last  week. German automaker Mercedes had a psychologist observe the money-men's reactions to the company's $250,000 Maybach vehicle, in one-hour sessions at Coopers Classic Car & ''Cigar. That's the new cigar bar on W. 58th St., owned by actor and Giuliani adviser Elliot Cuker. Appropriately, the Maybach boasted its own humidor, hideaway bar, electric slipper closet and an opalescent panel that lights up the car, bumper to bumper.