I have been considering something like this for a while, but I think I may have a new idea of how to implement it.
I've been wanting to build a Chorus or UniVibe clone into an expression pedal housing for a few days, but I stumbled upon the DOD FX67, the Stereo Turbo Chorus, which has two LFO's. http://www.americaspedal.net/fx67/
I think it would be interesting to re-house it into an expression pedal shell, and use a dual-gang pot to adjust both LFO's with the sweep of the expression. However, I wonder if it is possible to offset the rates, by either changing the taper of either gang, or by using resistors in series. Might be interesting to use reverse audio on one gang, and regular audio taper on the other...
Now the trick is going to be to find one of these little buggers.