I don't remember where I read this but I thought that it was Michael Stroka who was rumored to be cast as the hippie.
The DS Movie Book is definitely one of the sources that says Micheal Stroka was "a candidate" to play the Hippy, which I suppose means there may have been others under consideration - but, unfortunately, if there were, it doesn't say who they were.
We had a discussion on the forum once where it was mentioned that Tom Happer (1840/41's Jeremy Grimes) or Ray Carlson (1970PT's Aldon Wicks) might have made good candidates. (And an interesting aside about Carlson is that both versions of the DS Almanac say he played a Blue Whale customer in a '67 ep. I've never tracked down which one and I don't think anyone on the forum has ever spotted him either.)
I would have preferred to see Louis Edmonds as the very mod Gregory.
That could have been a hoot and a half!
Though the DS Movie Book mentions Michael Stroka or Keith Prentice as likely to have played Gregory. And we can get more into that once Gregory's scene pops up in the script/slideshow...