For completeness and similar to the Exquis above, is the Akuto Chord Machine. It is also currently going through Kickstarter.
The keys can be isomorphic, or a two octave standard arrangement, or as seven groups of four chord types.
A major difference though is that the Exquis has MPE in two dimensions. Far as i can tell the Chord Machine has none.
Another wild alternative keyboard by Bob Moog, this one reinvented and restored !!!
How do you notate music for something like this. Good on Cornell for doing this.
ADDED : It occurs to me, this is exactly the sort of situation, a recreation supported by a major university and some passionate creators, of a project from fifty years ago, by a person long deceased, that might be made at least partially open source. One could hope anyways.
AND MORE : Found another better article on this from CDM. The synth in this uses a divide down technology.
And even a better article :
