Interesting that they actually are midi codes, thought they ought to
be but Mike Nelson was
always somewhat evasive on this matter. (or maybe it was programmed
by someone else
and he didn't know, can happen with small companies).
This is what Mike answered when I asked about the MIDI implementation of the Boomerang: "While the Side Car(TM) controller uses a form of MIDI command, it is not easily duplicated by a third party MIDI controller. We MAY add remote MIDI control in a future software update as several players have asked for that capability."
Gordius or DIY controllers are probably the only viable options, unless a computer (or tablet, phone, raspberry pi) is used to process the signals.
I think your analysis and conclusions are spot on, although my own attempts at unlocking the hidden potential of the Boomerang looper have lead me to put it back its box. Any attempt at using this pedal in a way other than what its designer intended was quite disappointing. Most annoying were the inexcusable zipper sounds that obtain when the feedback is changed while recording or overdubbing. So much wasted potential...
I regret that I did not fully map out the midi messages from the Sidecar before selling it, but I think I have most of the commands mapped out in a file on my laptop...