Why don't you take another path? Buy a NOS Triumph Tiger 1050? There is a brand new 2010 one in the Seattle area for around $9,000. Get a subframe from Jay at DMC and install the chair you have. Lots of ponies and a very fine engine. May not get you down all the gravel trails you're accustomed to, but who knows? Maybe it will!
I love BMW's, heck I have owned three of them. But sometimes think that they are over rated. Heck even a newer DL1000 would work and allow you to get on gravel.
I'm guessing you will only be able to do this one more time before SWMBO sticks a trowel in your hand and points toward the flower garden. So why not try something new and radical and have fun with it?
Sold "Fiona" 1982 R100RT Hack EML/Squire ST2 chair
"Gracie" - 1983 R100RT 60th Anniversary Edition
"Seika" - 2013 DL650 + DMC M72D Sidecar
"Adventure is worthwhile." ~ Aristotle Airhead #14449