Back in 2001, an SVX here in Germany was outfitted to run on liquified petroleum gas. (Flüssiggas) The story is written up on a page at the German SVX Club (in German)
http://www.svx-club.de/Technik/Page1...page13527.html
He put a 70 litre tank in the trunk (see pic) and the total cost was around 2,500 euro. Seems to be happy with it, although it doesn't seem to have been updated since 2003. He says that for more info people should contact Subaru-Servicepartner Auto Wolter in Bergheim-Glessse, Germany 50129
The photos are coming out very big, so I've put them in my locker for folks who want to see more detail. Here are small versions with link to big:
http://www.subaru-svx.net/photos/files/Bipa/29118.jpg
http://www.subaru-svx.net/photos/files/Bipa/29119.jpg
http://www.subaru-svx.net/photos/files/Bipa/29120.jpg
http://www.subaru-svx.net/photos/files/Bipa/29121.jpg