I'd be interested in getting some of that kind of pressure sensitive ink for a DIY MIDI controller project, or a custom PCB manufactured that uses pressure sensitive ink (I haven't found a PCB maker that does that yet).

Do you have any links or info that could help?

I don't have much that can help. We started with pressure sensitive foam for our first proof of concept since it was much easier to obtain. Soon after we worked with a Chinese manufacturer to source a replacement ink. They silkscreen and cure the ink for us into the sheets we need as a component in the final assembly.

One thing I can caution you on is how finicky the inks are. Ink thickness, curing time and temperature, and abrasions to the ink can have large impacts on the responsiveness of the ink to pressure. We do a lot of software and hardware work to mitigate these inconsistencies.

Sounds a bit too finicky, hmm. From looking at teardowns of, eg, the Ableton Push 2, it looks like a gold-plated pcb with fingers that the conductive sheet sits on.