deskthority

I am interested in making some keyboards that work natively over PS/2. Has anyone here used a controller like the HT82K629A to do it? Does it support NKRO? How hard is it to program?

You don't program it. There's already firmware on it. You just have to wire it properly and it'll work.
Keyboard matrix is in the documentation of the chip.
http://www.farnell.com/datasheets/79209.pdf

You *just* have to use its one and only supported matrix. Which, in fact, could be a lot of bother.

The HT82K629A is a "Keyboard Encoder", not a general-purpose microcontroller.
It is not programmable. The block diagram shows a microcontroller, but it is probably running a fixed program in ROM. The matrix is hard-coded (but you wouldn't have to connect every key).
The datasheet says that it has "Phantom key detection" but no way to turn it off, which means that it does not support NKRO. The matrix also has the WASD cluster laid out so that there could be blocking there, so it would not be suitable for a gaming keyboard.
The USB it supports is only low-speed, and the datasheet does not even mention switch debouncing intervals, so I doubt that it would have very fast response time even over PS/2.

The ps2avr firmware for the AVR family of microcontrollers (which is in Teensy 2.0, Pro Micro etc.) does talk PS/2. It had been used for some early Korean customs, I believe. There should also be a USB version of it.

BTW. It you want a programming challenge: I think the ATmega32u2 (and ATmega16u2/ATmega8u2) is the only AVR µC that provides direct access to the USB lines, and thus theoretically able to do either PS/2 or USB over the same cable. But I don't think anyone in the community has programmed a firmware with support for both interfaces.

Interesting, thanks.

I found some PS/2 controller that looks better. Very easy to program firmware for it
http://www.sprintek.com/en/products/Keyboard.aspx