Just found that link in irc:

http://dclausen.net/projects/tfam/

A quote as teaser:

This is a Macintosh 512k which I upgraded to run OS X by replacing the innards with a modern Mac Mini and various supporting components, including a grayscale CRT monitor, an LS-120 floppy disk drive, and a microcontroller-based USB device that interfaces the Mini to the original keyboard and mouse. Why? Mainly because I wanted to experiment with creating a custom USB device. Also I guess I wanted to waste hundreds of dollars and countless hours building a semi-useless computer with a 9 inch black and white screen and no arrow keys.

A video seeing the Mac running:

Tags: 24th Anniversary, 512k, MacMini