I’m excited to see this project IC, and follow the developments!
I think @norbauer will be terrified(/delighted?) that you just removed his primary concern with doing another round of the Heavy-6. I can imagine an appetite for PC Heavy-6s specifically, due to the LEDs, and perhaps interest in other finishes as well for those who stray from the Topre path and are compelled by the MX compatibility 
For the true Topre-devotee: did you explore ways to add similar LEDs to an actual FC660C (likely through a HASU-esque custom daughterboard)?