Jacques,  I am also the owner of an early vanagon ASI camper (1980 air cooled model)
And I've often been curious and wanting of a rear tent 'extension' I've been planning on making my own this year
Documented here are similar ideas ( http://www.supernet.net/~jclark/vw/loaves.html )
and this one ( http://www.kookymathteacher.com/rearender/rearendtent.html ) 

My plans are to make a 2nd tier parcel tray of 2 13mm alum C tracks "[" fastened on each of the side cabinets at the height of the rear bench seat back. using a piece of quality plywood that'll slide into them for 2tier stroage while traveling. but slide out and be used to make a bed extension while set-up for camp. 
for the 'tent extension' fabric I was going to goto a local Marine canvas (for boat canopies) andhave it custom made. 
but PLF ( http://www.plfvw.com/home/default.asp ) may be able to help you with one. 

There aren't many van Rivieras on the Eastcoast, does the Quebec camping club ( http://www.westfalia.qc.ca/ ) welcome the ASI as they do the Westfalias? 

Dan Stevens
1980 ASI riviera with Turbo Diesel converison
______________________________________________________
From: Jacques Pag� <jacpag@videotron.ca>
Subject: Rear extension on a vanagon Riviera (almost a westy!)

Hi to all,
Last weekend, I saw a camper with an extension made of canvas that was
installed in the back of a SAFARI CONDO camper (based on a GM Safari
van. It was supported from the top of the rear hatch and had a bottom
panel made of wood, with all the rest made of canvas. It did not have
legs that were touching the ground.

This thing was very intereting and it gave me ideas for my own camper,
a 1981 Vanagon based RIVIERA camper. This camper is mostly like a Westy.

My question to the group is:
Does somebody know about such an extension made for the Westy? Or at
least, plans to make one...

Thanks in advance to all of the group and I wish you all a very happy
camping season.

Jacques Pag�
Montr�al Qu�bec Canada