First I thought about integrating a LED with the buttons and bought some very standard switches allowing me to do exacly that:
I built a four-key keyboard prototype on a stip of vero-board. The prototype scans rows of keys one row at a time and returns the result on a 2 bit bus... The same bus is used for turning on and off the LEDs connected to the keys. The LEDs also use line scanning, so even though all leds in the picture below appear to be turned on at the same time, only two and two are.
To achieve row addressing, a 74HC154 demux ic is used. The bus is shared through a 74H08 AND and a 74HC125 tristate buffer.
The vero board also contained a midi interface, connecting to the USART on the MCU. At the time I had no real schematics or PCB editor, so all design drawings were done in Adobe Illustrator!
The prototype worked very well, and allowed me to identify a few logical mistakes.