We had frames custom built for our R80ST and R100GS by Ernie at
www.overland-solutions.com. He did an excellent job and strengthened the sub frames as they are prone to cracking with the extra weight of panniers. He can also supply Touratech/Darr panniers or will probably be happy to fit them if you already have them. He can also build racks, tyre brackets, etc if you want those.If you want to see his work, look at our web site
www.chasingthesun.org under 'bikes' - there is a link to Ernie's web site on the home page.
Ernie is based on the Isle of Sheppey in Kent and it takes him 3/4 weeks to manufacture the frames. The cost is about the same as the standard Touratech frames, but they're much more robust and have survived a couple of tumbles already!
Hope this is useful and good luck.