I want to bump this thread, and link in another thread on an interesting hexagonal MPE controller, that may be implementing a microtonal interface as well.
There has been some discussion in that thread, with a developer, on implementation of microtonal MIDI controllers.
The Exquis from Intuition Instruments has 54 hexagonal keys each with two MPE dimensions. So it’s not a Linnstrument, or a Lumatone, but it doesn’t cost anywhere near those either, and has a very small footprint too. I like that it has CV as well.