Im sorry that your local club doesn't allow rain tires. WTF is with that, anyway? If you ablolutely MUST race in the rain w/o rain tires, just make sure you apply power slowly, and brake early. That's really all I can say. Take a rain race as an opportunity to learn how your bike handles in a slide. It's kind-of like racing on dirt. If you learn to drift your bike really well in the wet, and apply it to drifting it in the dry, you CAN use it to your advantage.
For example: The first time someone 2-wheel drifted past me in a turn, (Eric Haugo, AMA Formula Extreme # 9) I was totally freaked out. I assumed that he was insane (because he was TOTALLY crossed up) I did not want to go near him for the rest of the race. So, lesson learned, a big fat 2-wheel slide can be very intimidating for the competition. I'm sorry that this is the only advice I can offer. You should really petition your local organization to allow rain tires. I really think it's rediculous that they dont!