I wonder if the episode would have worked the same if the producers had used real 1960's era musicians as the Space Hippies. Maybe Frank Zappa as Dr. Sevrin?
The Beatles, maybe? I wonder if any of them were Trek fans.
Which Beatle would have played Dr. Sevrin?
Dunno...I am actually curious to know if any of the Beatles has ever said they were a Trek fan. I know they were all huge Python fans (Ringo was on Monty Python's Flying Circus, and George was in Life of Brian) but I'm not aware if any of them were Trekkies.
If I had to pick one though I'd guess John would have. He was the most political of the four, wasn't he? So the role of Sevrin might have appealed to him, since Sevrin definitely was a man of strong opinions...