Is there any way to turn off the automatic headlamp function on the 2000 Chevy Venture other than when it is in park?

I had the same concern until I read in the manual for the car. What you do is ----- before turning on the ignition, you press down on the park brake pedal (not the regular brake, but the one that you press after you are parked). Then you turn on the ignition. When you are ready to move, remember to take off that brake. It works. Now I can enjoy the warmth without having lights on.

If you want to drive the car without the Daytime Running Lights you have to remove the fuse (or leave the parking brake engaged...).