@Trekky, it is working using the Adafruit RGB Shield as a test platform. First, you can’t connect the INTA output to D2 of the Core as it is not 5V tolerant (!) On the shield, I had to solder a wire from pin 20 (INTA) of the MCP23017 and connect it to pin D3 of the Core. I also found a little oddity in the RGBLCDShield library which used pinMode. It turns out that the Arduino definitions for INPUT and OUTPUT are opposite those for the Core!
The combo of both libraries required to use the RGBLCDShield is here. I included the HelloWorld example and also the interrupt example which shows the use of interrupts from the MCP23017. @Trekky, you can modify interrupt.ino to test interrupts without the shield easily.
@JackANSI, if you want a working RGBLCDShield library, there you go!