I have a standard bike bell mounted to the back of my seat post. It took a bit of practice to be able to find the lever while riding, but it works quite well, and has helped me avoid more than a few collisions.
That's a really good idea! I could also see one being mounted under the front of the seat being a convenient place to reach -- but maybe just a handheld bell (maybe one that strapped to ones' fingers?) would be best.