I performed an experiment based on this idea - I wired up a clicky relay to the numlock LED (via a transistor), and glued it to a decent Dell keyboard. Then, I rigged up a python script to toggle the numlock led on each key press/release event.

Honestly, typing experience wasn't too different to a real mechanical keyboard.

Good mechanical switches have a kind of mechanical haptic feedback that lets you know with a click, that your keystroke has registered, without bottoming out the key.

Right. I wonder if travel can be eliminated even further with vibration-based, rather than actual mechanical, feedback.

That sounds pretty expensive, IMO. There would need to be one motor for each key that's isolated so only one key vibrates. And on top of it, it would be a pretty big keyboard too.

If somebody has done this before, I would love to try such a keyboard.

I'm not sure if you can get enough force from them, but maybe a piezo crystal under each key could provide localized feedback more economically and with less space requirement.