The model of the Roland/Boss expression pedal is EV-5. As I remember it wasn't really expensive.
I have amps galore here at H101 headquarters and using the stereo out on the RT-20 was great fun. I in no way meant is sounded crappy with only one amp in mono. It just sounds better with two if you have a second amp. Note: the second amp doesn't have to be big and powerful to add to the rotary effect.
Hammond Wiki has a line out you can build that will accomplish your switching needs. Build the circuit with the volume pot and you will be able to pad the level down to something the RT will like. I also feel it is not neccessary to defeat the speakers in your console. If running only one amp for the rotary it will add to the effect. My 251 sounds much deeper with my HR40 running at the same time. I found this true with the RT-20 as well. Great chorale.