"@Don Selesky: Also Serenity Shelter from MLD, with all-silnylon front+back, could nearly be a solution... only need some more protection on the side (not much more...5-10 inch)"
Yes, the Serenity Shelter was what I used last summer. Luckily I didn't need to deal with driving rain, but I had my tarp set up low enough, in a sheltered area, that it probably wouldn't have really been an issue anyway. With the tarp staked low, the inner really isn't that exposed (assuming you have a reasonably sized tarp).
If you want even more protection against wind-blown rain, you might want to go with their Superlight Bivy. The current one runs +/- 7 ounces in Silnylon, and breathes pretty well. Plus, it adds extra warmth.