There's a British single-seat Longeze derivative called a Goldwing. I've flown them a little, but never in rain - the aircraft carries a large and clearly visible placard stating "DO NOT FLY IN RAIN". The manual states that this is because of the rain breaking up the laminar flow on the mainplane and canard.
I've spoken to one pilot who did make this mistake and said he could just maintain level flight after passing through light rain with full power and full back-stick.
Some reasearch was done into this particular aircraft by Glasgow University in the mid 1980s. I believe that they concluded that although technically both mainplane and canard were designed for laminar flow, the real problem was the canard. I've seen a report from them that stated that a suitably rough paint finish on the entire canard would prevent this problem and allow flight in rain. I'm not aware of anybody who has ever felt the urge to prove this in flight-test, I certainly haven't.
G
