I'd second the idea of going with a slightly wider rim such as the Mavic 319 and use 28mm tires. It sounds like you could use the extra padding of the 28 and the wider rim that it would sit on more correctly. In effect you'd be buying or building a set of cyclocross wheels but using road tires of suitable width.

And given the nature of the weather in Seattle, which isn't any different from where I am, I'd look at using mountain bike hubs just for the better sealing to try and keep the water and muck at bay. You may need to fiddle with the spacers and axle to fit your frame's rear spacing but I think it would be worth it. XT hubs are a good bang for the buck on that count.

Radial isn't all that bad but if I were building a set if wheels for the sort of conditions and usage you're describing I'd opt for good ol' basic 3 cross on 32 hole wheels. Or if you regularly carry a lot of heavier loads on a rack go 32 on the front and 36 on the rear.... or just go 36 on both ends. The few grams extra weight is diddly when your goal is a long lifespan of aggresive use under difficult conditions.