I have a feeling that the vast majority of pilots would be happy to speak to a passenger who is fearful of flying. My strategy would be to speak to a flight attendant as soon as you get on board (before going all the way back to your row), let them know your concerns and have them ask the captain if it's OK for you to come up and speak to him.. sometimes the pilots are out milling around the galley area near the cockpit before the flight and you might even be able to address them directly... or if you're at the gate early enough you may see the captain and first officer as they arrive at the gate and you could approach them at that time.

I would not start off by saying you're not going to fly unless the pilot comes to talk to you as that sounds like a threat and it could cause problems.