I bought the panniers used from a guy in Colorado and he sent me what he said were the mounting brackets. I have a feeling the brackets were meant for an R100GS though as the rear mounting point didn't line up with anything..... so I just took it off and have them mounted a little less securely. After a few trips with them heavily loaded I haven't had any issue.
if you send me your email address I'd be happy to send you a couple of pictures.
your WIP GS looks like it's gonna be a great bike!