My basic idea is to take two identical computer keyboards and combine the transparent switch matrix sheets from both into one so that there is a time difference between the upper and lower once being triggered.
I see some problems with this, but I'll start with just one pair of switches and see if I can get a measurable and consistent delay between them with which to determine velocity.
If that works, I'll next try combining the usb connections from both keyboards and see if I can get two sets of key-press info and measure the differences between them. That would be ideal for making an inexpensive kit for DIY peeps. That would include a set of 3D printed key caps for various kinds of isomorphic keyboard layouts, like the one in my video above.
If I can get away with the USB trick, that would let people build using any pair of identical usb keyboards.
Another approach would be to just buy a stack of identical cheap keyboards and convert them with their own internal scanner and MIDI and/or CV out, all built-in for sale as a single unit. If there is space for a tiny synth inside as well, I may have the microtonal casiotone for the world dominations ...
A more expensive, but probably nicer approach would be to convert a cheap midi keyboard to add a second row of those rubber key switches for a more 'normal' keyboard, but with an extra row of keys above the first to let you still reach octaves in 19TET and other scale setups.