I been looking for the exact same thing for exactly the same reason.
The brim design on MTB helmets works for not catching the wind, but it's way too skimpy. Hardly a brim at all. What there needs to be is a bicycling helmet that's identical to a pith helmet in shape and size. Or maybe the full-brimmed style of hard-hat. Make it safe enough to work for bicycling and give it vents in the same manner as an MTB helmet.
The best thing I've found so far is DaBrim's Rezzo brim. I use it on a Kali Lunati Sync helmet. I don't like the full-brim products by DaBrim, as they are too big, there's nothing to keep the brim from flipping up in the front (AFAIK), and they get bad reviews, concerning how the too-big brim ocillates in the wind. I don't want to wear a brim that big. A brim that sticks out 2.5" from my head is the minimum needed to shade my face, and that's as big as I want to go.