Actually, rain shouldn't be that hard to pull off. In real life, you don't see the actual droplets in the air until it hits your windscreen... unless it's really pouring, and then it's a moot point because you wouldn't be flying then anyway.
You could make the rain effect by an animated graphic on the windshield... which we have now in the form of oil and blood... and you could make the reduced visibility by using a thick grey fog... which we now can do in AH2.
Apache/Havoc -> Comanche/Hokum did this exact effect really well, and those games are 3-4 years old.
I think the biggest problem is that people would whine and log off, for the same reason they do at nighttime.