There's a multitude of tablet and computer apps that will do a simple rechannelization and send the MIDI back to the arranger. MIDI Solutions boxes are an expensive way to do a simple task...
I'm not sure I understand the need for non-transposing style parts unless it's for percussion, though. In which case, creating an extra Drum Track achieves the goal without the need for transmission and rechannelization.
There's another way to create non-transposing Parts on the BK9, though. Roland's Key Audio feature allows you to record a loop of any thing you want, and play it in sync with the arranger. You get up to eight per Performance, and can turn them on and off either off the top few notes of the keyboard, or using the FC7 input.
For me, I think that's a lot faster a way to get breakbeats, percussion, drones, sfx etc to work with a style than writing Style Drum Parts for all the Divisions and going through all the rigmarole of sending, rechanellizing and returning them to the arranger. Plus you aren't chewing up a whole Style Parts for an effect or whatever. Remember, every Part you turn into a non-transposed Part is one less Style Part for regular stuff!
Nowadays I'm primarily gigging with audio backing (the guitars are real guitars!) and only about 20% SMF's, very few styles. My playing style doesn't really like restricting my LH to rote chord playing to drive the style engine, and the Chord Sequencer has to be loaded up manually, so too slow for a live gig...
My current project is converting all my backing to audio files, and then playing them from a tablet. This way I get synchronized lyrics and the Mark/Jump feature on audio tracks (BK9 only can do that on SMF's, not audio) so I can extend or shorten songs depending on circumstances.
One of the original strengths of style play for professionals was the ability to repeat solos, add sections, end early etc, but Markers in audio achieves that and adds completely realistic guitar parts and things like backing vocals, real drums, percussion etc.. Technology has done a good job of making a lot of the original reasons we chose arrangers somewhat moot! Possibly why the segment is in such decline...
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An arranger is just a tool. What matters is what you build with it..!