Dual Pro Micro's. One running TMK, the other running an Arduino sketch driving the LEDs. Pro Micro's are connected to each other via hardware serial UART.
\tmk_core\common\keyboard.c was modified to send the row/column of the key being pressed encoded as as single byte over the serial link to the LED controller.
#ifdef GNAP_ENABLE
//send single byte with value of row column
serial_send((r*16)+c);
#endif
The example Arduino sketch [LED_FastGPIO.ino](LED_FastGPIO.ino) uses the fastGPIO and TimerOne libraries. These can be installed with the Library manager.