Due to the extra leverage required to apply braking-force near the center of the wheel, disc-brakes uses a significant amount of force on the pads. Something like 5-10x more than what's applied to the rim in order to get the same leverage. So that extra pressure effectively squeezes out any water.
I'm not sure what the big deal with disc brakes on a road-bike is anyway. I've done a tonne of crits in the rain, close to 100 and I've never had any problems with brakes. If anything, they're less important in the rain because there's less traction between the tyre and the road, so less total friction available. The brakes only have to generate enough to lock up the tyres and that's it.