While watching the YouTube video of "Stand by Me" from Playing for Change, I noticed the instrument that Washboard Chaz was playing. The picture is a screen capture from the video so the clarity isn't great. Looks like he has two cans wired into the sides and a wooden sound block at the bottom. You can also see a desk clerk's bell over on the far right of the setup. If you head over to his website, you can hear some more examples. Listen close and you can hear that bell ring out occasionally.
I messed around with using a washboard as an rhythm instrument a few years back but felt like it was too loud. It overpowered everyone else.
I may dig it out of the basement and add a few extra bangles to the thing to see what sounds I can come up with.
Update: I did a little more digging on washboard playing and found a DVD by David Holt called Folk Rhythms. I had two ah-ha moments while watching the DVD. You are tapping on the washboard more than you are sliding across the ripples which was what I was doing all the time. By tapping and using short drags occasionally, it softens the sound. My second realization is that by adding the extra goodies at the bottom you essentially have a hand drum with three distinct sounds. Which means, I think, you could apply some of the same principals of playing a frame drum to playing a washboard.
[Image: Captured from video "Stand by Me": http://www.youtube.com/watch?v=Us-TVg40ExM]
[Image: Captured from http://www.washboardchaz.com/links.html]