The rear wheel of the 6 is a "dust bucket"! There's no denying that. As others have stated the wheel is rather obscure and not as noticeable as other cycles and if you have a center stand you can access the spoke cavities for cleaning but you'll only have access to the bottom portion of the cavities as the brake rotor pretty much hides the remainder.
000_0454.jpgDSC_0007.jpgDSC_0002.jpg
If you truly want to remedy what you perceive is a problem, you could fill the spoke cavities with "rigid foam sheeting inserts" and secure them in place with some black Silicon around the perimeter. That will take care of the cavities and should have minimum impact on wheel balancing. Use "Ride-On" tire sealer/balancer and you could eliminate any wheel weights. Just a thought