I think you Glitter boys are approaching it from the wrong angle
I also have a XCountry, which has little or no rear wheel protection and BMW have produced a number plate extender for £14
Without it, the whole of the rear seat/your legs/any pillion/rack & bag and lights get covered in water and road muck
However with it fitted the end result is that the phenomenon is 90% reduced
I was out yesterday in the Dales on wet and muddy minor roads, so decided to take some pics for you all
The problem
A probable solution
The clean result - a clean exhaust/rack/bag and seat
I also have this on my 1150Adv and the rear number plate deflector, works effectively well (I have followed many GS's over the last 20 years in rain and these do work)
So there you have it, fit a rear number plate deflector of some description and you will halve your muddy/legs/dirty pillion

and soiled luggage problems
Conclusively they work on GS's