I resumed calling local Avis offices this afternoon, and while on hold on my third call (for 23 minutes 51 seconds) (seriously), a co-worker stopped by my office. She made the mistake of saying "How are you?" so I let her have it, and told her all about my experience in Avis Hell. It turns out that she used to work for Avis! She was still in contact with a current employee, and she put him in touch with me. He tracked down the previous renter, and he's going to contact her tomorrow morning to ask about the camera, and give her my contact information.

There's a lesson to be learned here. They say it's better to light a candle than to curse the darkness; I spent four days lighting candles in vain, but when I started cursing the darkness, everything fell into place. : ) Thanks so much to both of you, not only for your suggestions, but for motivating me to keep trying and for reminding me of why it's important.