Hello Jim
First of all sorry for my english. I´ve been doing what you ask with my PSR 1500, my Midi pedalboard and other keyboards and it´s quite simple. You have to edit a Midi table in the arranger. In the RECEIVE section of the midi channel matching the pedalboard sending channel you select the part of the style you want it to play. I use the bass part and it will change the bass sound every time I change the style. That is what I want. If you want the pedalboard or midi controller to play a fixed sound/s you have to set the receiving channel/s in the arranger as SONG part. You can make your own selection of sounds for the controller/s in the arranger´s mixing console (song part) and save it as a registration (select only SONG in the Registration Memory Contents if you don´t want to change anything else).For the pedalboard I recommend you set the bass recognition (on bass note) in the midi table (matching the pedal sending channel) to change the style bass note playing according to the note you touch in the pedal. These Yamaha arrangers have 32 channel simultaneus playing capabilities so you con make it play many more parts at a time (not only 2 or 3)using external controllers.
I´ve always wanted to recreate an organ using an arranger since organs are exctint in my country, or extremely expensive if I import one. If you don´t understand or have any question let me know.
Victor