How's about THIS:
A pedal that is a simple volume and tone control, just like your guitar would have, to run before your ODs for those times when a guitar-turned-down-into-an-overdrive sound is what you're after. That way, you don't have to fiddle about with the guitar's knobs, you can set the controls on the pedal and just switch it on and off.
Hmmm...
That's essentially how a volume pedal works. You can also build passive dual volume pedal, see the beavis audio tech pages for that.
But the tone pot thing is iffy. You would probably want it to be active, and an eq pedal might be more what you're wanting.
Jacob