This thread reminded me of what 'vintage' projector Sylvan Muldoon wrote about it. He was an experienced RTZ-only projector, so he might fit the hypothesis. However, here is what he says and I think that it clearly contradicts the proposition that it wasn't widely done by people within the 'occult' or 'medium' scene back then:
Read downwards from "As far as higher astral conditions are concerned"http://books.google.de/books?id=Z3hRU7_mCVEC&pg=PA179&hl=de&source=gbs_toc_r&cad=4#v=snippet&q=%22So%20far%20as%20higher%20astral%20conditions%20are%20concerned%22&f=false
So, in a nutshell, Muldoon mentions occultists or mediums who project to the "higher planes" all the time and he was even told that he might not be 'evolved' enough because he couldn't do it (as he admits). So this view would back up the argument of 'evolution' when we start to reach higher planes too. (I myself can confirm what others write here: I started to have RTZ projections and after a few of them never anymore).
Regarding older literature, please remember all the Theosophical or other occult books by Leadbetter, Blavatsky, Besant and others. They drew maps of astral / mental planes and bodies. How would / why should they have done it if they had't been "there" experiencing it for themselves. It is true that they derived a lot of their 'cosmology' from Buddhist or Hindu teachings already, but they were avid experiencers and themselves, like many secret societies then.
I don't believe that for a second.
Granted, I haven't read any of his books yet... but it seems to me that *EVERYONE* has projections not in the RTZ. The difference with Muldoon (and let's be honest, with most people who claim to have nothing but RTZ projections) is that he didn't recognize them as such, so he probably greatly ignored those projections. I see this happening on a massive scale from new people who come here to the Astral Pulse. People completely ignoring valid experiences because they don't line up to someone else's expectations or what someone else has told them it "should" be like.
In this case, Muldoon was Muldoon's worst enemy... and it shows that we need to drop two things in order to fully free ourselves to experience: BELIEF and LABELS.
Drop those two things and just EXPERIENCE, and you'll open yourself up to everything reality has to offer.