SNIP
The most portable arranger I can conceive of is a 32-key controller for the right hand (F1 to C3, probably 7/8 size keys) and a similar two-octave keyboard for the left hand, arranged "organ style." A foot switch or MIDI foot controller could take care of voice and program changes. You could almost play this while sitting in a coach-sized airline seat!
The keyboard controllers would drive a BK-7m or SD40 module, perhaps vArranger on a laptop, etc. The whole thing would have to run from some kind of battery pack, with voltages tailored to the various hardwares. Build it and I'll buy one!
I agree to your idea of the most portable arranger. I use Korg Microkey Air, vArranger on a touchscreen, and a wireless USB foot controller the Pok by Xtempo.