I was about to date myself and say "there's probably a dcop interface exposed"... But there's probably a dbus interface exposed. You could write a script and plug it into a cron job or similar to tell it to adjust the background every hour or so. There are some nice dbus exploration tools available so you can discover the correct call to make.

If that doesn't work, the hue is probably stored in a human readable file somewhere in your .config folder. You could probably have a script adjust the value in the file directly. Might take plasma a little while to notice, but it likely will.