Actually I just figured it out. Simpler than I thought. I was looking for the buzzer (I ended up finding it above the trunk release, but it's really hard to reach and would be tough to reconnect), but I should have just looked at the wiring coming from the seat belt latch. The new Smarts have a simple connector that you can wire to itself (
Evilution - Seat Belt Reminder Alarm), but the old G&K Smarts can just be unplugged. Slide the seat forward, pull the wire out from the carpet, unwrap the tape, and unplug both plugs. This way you keep the door buzzer working, but the seatbelt buzzer is totally cut off. Easy to reconnect, too.
Thanks Jim for the suggestion, but the old pre-2008 Smarts don't have park, the buzzer buzzes even when the car is not running, just on. It's awful. The importer put the most annoying, loud, high-pitched buzzer in the world in there, too.
So anyway, if anyone searches for this and finds it, I'm gonna be the (2nd) to say "Don't drive with your seatbelt off!" Only do this if you often find yourself working on the car with the power on, or you leave passengers in the car and don't want them to not be able to hear the radio or use A/C. Always wear your seat belt, even if you're just driving down the street. You never know who's gonna not be watching the road and crunch up your little Smart. If you can't remember that without a buzzer, then don't disconnect it. Drive safe.
-hoov23