Hey David...well, the RS2 uses a great coaxial cable, shileding around the piezo, and pro audio shielded connectors...so just about everything is being done to keep it hummmmmmm free that we can do. I am talking to someone now about anything else that could be done...but I think it's pretty bullet proof as is.
Fishman used to include a grounding wire...but now they don't have to as they mount the jack on a metal clip that goes on your strings. If the clip is on the metal of your strings...and not the thread...then as long as your hands are on the strings even a dirty ungrounded plug in a room full of neon signs would prevent it from buzzing...that is until you take your hand off the strings.
I would say a length of pliable wire in your gig bag could be a good idea with
ANY pickup if you are playing out a lot and in unfamiliar places...that way you never get caught off guard.
...I guess you can attach a grounding wire looped around the RCA jack and tie it into your strings...this is similar to what Fishman used to provide. This maybe your only help for a quick fix. 
