I've seen pedals that use an LFO with a switch that would replace the LFO with a picking dynamic circuit . Seems like putting the dynamic picking part into a separate enclosure would be something that could then be tuned to use with any effect that had an expression pedal input. There was a similar add-on project to build into a couple of the effects in Craig Anderton's Electronic Project for Musicians, but I don't think I have ever seen this done as a stand alone box.