Wunderlich has a new fender part, which seems useful to me. It is advertised to reduce dirt flinging around from the open rear frame area. Google translation straight from the Wunderlich website:
" Who was not already annoyed. The free framework triangle of 12 GS models does not offer protection from splash-water and the dirt whirled up. The water hurls unhindered over the boots and up to the thighs. Substantial dirt exposure of driver and pillion are the result. Plus, the dirt arrives into the most remote corners under the seat, and labouros cleaning is required thereafter. Our interior fender eliminates this problem in an impressing way. Inconspicuously it integrates into the bikes design. It fills the gap in the otherwise open framework triangle, and provides a uniformity to the framework concept. "
This part is made from ABS plastic, cost is Euro 130 / US$ 200.
source: German GS forum http://www.gs-forum.eu and Wunderlich