The first time I did it, I used the seat/rack from S. Meyer intended for the R65-R80G/S. Â It did not pivot, the rack had to be welded to the subframe, and I never did get the seat anchored properly.
Also, Motobins offers two different G/S and GS rear racks (scroll down to Seats & Seat Parts):
#91931 and #91932 Â http://www.motobins.co.uk/bmw-parts.php?model=R%20Series%202%20valve%20Twin
I finally bought the proper "police" seat for the R65 (it works best with a '81-'84 bike, but a early bike can work with it with a later subframe).
It pivots and latches just like a normal bench seat. Â Unfortunately, I don't see them in S. Meyer's catalog, but it is certainly worth an email to them. Â They readily trade with the U.S.
Here is the link to the G/S seat/rack:
http://www.meyer-bikes.de/index.php?page=details&art=52531453837SKThis is a link to my project photos:
http://www.pbase.com/tomfarr/solo_seatIf you go the police seat route, you will need a couple of special parts.
The pivot and the special tapered nut.
I have a picture of the pivot, but not one of the nut, except in situ:


the hinge and the nut are not cheap, and A&S calls the nut "OBS"...
(thank you RealOEM!)

Hinge is #7 and the nut is #13
07 Hinge  1  52531236948 $30.05  ($26.44 A&S)
13 Nut  1  52531242706 $10.53  ($6.48 A&S)
When I used the G/S/ seat, I had to weld small plates to my subframe to contact the rubber bumpers. Â

I recommend the police seat if you can find one.