Programming the MCP23017 - set gpio "LOW" instead of "HIGH"

Hello All, I am using the MCP23017 port expander to detect button presses. This code came from Nick Gammon's helpful post:

This works fine for a button that takes the input pin to ground. I am trying to use a 5 button membrane "keyboard" - but the contacts from the common pin to whatever button is pressed has considerable resistance, and cannot pull the gpio pin low enough for a good "ground" signal.

I tried not activating the internal pull-up resistors, and not inverting the polarity on the registers, but I do not get an interrupt signal from pin 19 on the MCP23017.

What is the proper way to configure the MCP23017 such that the gpio pins are set "low" and create an interrupt when the gpio pin gets a "HIGH" signal.

Any tips will help.