I have to reply to myself because something very unexpected happened. I discovered the supposed existance of the GS-1 back adapter mentioned above. I did quite a bit of searching back then and concluded that it was a unicorn, and very few (if any) were actually made.
To get the KEH link in the above post, I had to search using a general search engine (i.e. Google, but I use a different search.) When I looked at all the links in the first page of hits, all of them were that purple color of links that had been visited, except one. So after I posted above, I visited the blue link that I hadn't visited before.
It was a camera supply store in Hong Kong. They had a sale on Linhof gear, with some items on clearance. They listed this item for HK$660 marked down from I thin HK$3800--US$85 marked down from just under US$500. I emailed them, and they still had it. I paid, and its on its way to me now.
Unfortuately the GS-1 backs are 6x7, and I much prefer 6x9, but I already have 4 GS-1 backs, and they are much smaller than the Linhof Rollex or Super Rollex. So I'm kind of stoked that the adapter may still be a unicorn, but at least it will be my unicorn.