I've wanted to do this too - for animated desktops. (You, you can have animated desktops by running the screensaver in the background via the terminal, but that seems to cause compatibility issues generally).
Your closest bet might be to use something like the Desksaver Plus widget to run the screensaver as the background. Then, if you're using 10.4, you might be able to use Quartz Composer to create a screensaver module which changes every second.