My rear wheel is a complete mess thanks to previous owners who were too lazy to wash them dowm occasionally, so I have been looking into the powder coating idea. I'm going to be going to my in-laws in Worcestershire at easter, so going to give Central Wheel Components are go.
Had an email reply from them today - I asked about spokes and rebuilding etc:
"The GS carries the BMW Tubeless wheel system where the spokes are tightened at the hub, as yet we have been unable to rebuild these wheels as the process differs from the traditional method. So to powdercoat your wheels, we would have to powdercoat the whole wheel, this would include the spokes. The full price for powdercoating the wheels would be £58.75 per wheel including the VAT, or £70 per wheel if you want us to remove & refit the tyres as well."
Seems quite reasonable - just hard to imagine how the powder coating of the spokes will turn out.