I tried rain-x on my prescription glasses and it didn't work out at all, so I modified an old baseball cap by cutting the rim off and leaving a bit of the forehead section above the rim which attaches to the velcro on the helmet (rather loosely, but is held in place by my head). This worked great since the ventilation wasn't affected at all, and in heavy rain, I would cover the helmet with my rain-gear hood.
Unfortunately, I lost the rim so I can't show you how it looks/works.