Is there any historical information available about participation rates - especially back in the "glory days" of the mid-80s?
Nothing terribly reliable that I've seen.
I know that there were well more than 300 gamers in Anchorage in 1992. Why? Because someone had that many members signed up for a local mailing list. I wasn't on it, nor were my players, but some gamers I knew were on it.
And I know that, as of 2014, Chugiak HS, my alma mater, had a group in front of the old library on the ramp (it's now a classroom pod, and the ramp's 5m wide), a group in the Social Studies center, a group in the Band room, and a group in the ROTC room, all playing at lunch. While the band and ROTC rooms aren't in the same locations, it's the nature of the space. the SSC and the ramp are both locations that are unchanged. A 5th group was playing just outside the staff lounge; that location didn't even exist when I was a student; it's in the expansion that was started when I was a junior.
I know also that East Anchorage High School had at least 4 groups - two in the bridge between SWS, one in the choir room, one in the commons, and one in a classroom. I know that, in 1986-87, there were groups playing in the bridge between SWS... both from days spent on field trips to EAHS, and from friends in those groups... And I know someone who ran a game in a classroom in SWS.
In fact, in all the high schools I spent time at in the 80's, I've found groups in similar locations during my time as a sub. Similar group sizes, too.
In no small irony, at least one of the groups at East was playing using books older than themselves... AD&D 1E. (Other games I saw at east include Rifts, and D&D 3.X.)
The D&D club at Eagle River High had 30 members at one point in 2014...