I would ask for help on the BMW forum here on HUBB and over on ADV rider forum. G-spot or regional forum perhaps?
I lost a starter on my old R100RS and managed to ride from Quebec to San Francisco with NO starter. Two weeks worth of bump starting. Park on hills!
Pray for warm weather! I got pretty good at bump starting this fully loaded beast. The big GS is actually easier due to up right riding position. Easier to run and jump on it.
But its heavy and stupid huge, so be careful if you've not done this before.
Is there something in the ABS, CAN bus or BMW system that disallows bump starting? Sounds like something the Germans might due to be spiteful.
I'm only 5'6" and I managed to bump start my RS but I grew up on dirt bikes so am good at bump starting.
In 2004 I was riding with a friend on his R1100GS in Mexico. His battery went dead. I bump started his bike for 3 days of riding until we found a battery in Ensenada, BC. He couldn't bump start it himself so I did it.
Really not hard. Its technique, not technical. Hills are your friend. Keep it running at Gas stations.
Good luck! Once you find a part somewhere, bump start that beast and ride to it. Could have some waiting around and hotel money?
Good luck, I'm sure the GS's Zealots here can find you a starter. Maybe something off a WW2 Ural?
Or VW car? Prost!