Hi, Peter @psirca, what you’re aiming for will likely involve the Dispatcher pipe and virtual mappings to achieve it.
It has some similarities with this recent question which has this patch as a (first stab at a) solution.
That might be a good starting point to build your own.
Be sure to look at the different modes of Dispatcher; you can use them to dictate whether the channel order is dependent on Note On order or Chord order
(PS. So all your held chords will have exactly 8 notes?)
is MIDIHub extensible via some API defined by yourselves
No, Blokas have not indicated recently that they have any plans for this.
Having said that, firmware updates are full of improvements that have come directly from users such as ourselves.
...
Perhaps the new phrase will be
There are some things Midihub can’t do; for everything else there’s pimidi!