Ah, I see what you want.
Scotch Guard is good, but its only hydrophobic, recent advances have given us Super-hydrophobic which doesnt just stop water getting on something, but also repels the water with force. If hydrophobic is a metal fence, super-hydrophobic is an electric metal fence with bared wire.
What you want to do is order some super-hydrophobic spray from America and try it, take the bellows off of the accordion completely and give the inside/outside a quick spray coating and just let it dry out naturally as suggested above. This means any rain or water that should get on it will run right off, the spray actually repels water so water grabs on to the dirt and pulls it off via momentum.
I dont quite know how it would affect the bellows, it could stiffen it up a tiny bit but Im doubtful of that. The only way to know is for someone to try it out and let the rest of us know how it feels to extend and close.
Kem