So there are 3 published libraries for the MCP23008, two of them seem to be separate efforts to port the adafruit library with pretty much identical results and the 3rd (arguably structured in a nicer fashion) built from the ground up by @rikkas7. The adafruit libraries currently lack support for the interrupt pin and the -RK versions (when I last checked) have neither support for the interrupts or reading all 8 pins at once.
I noticed the Adafruit MCP23017 does have the interrupt features so I cut and pasted those into a local copy, ripped out the port selecting parts and behold that seems to work, its not exactly elegant as the 23017 and 23008 do not appear to be written in the same style despite headers claiming the same author(s).
So I guess the question is do any of those in charge of maintaining those libraries want my sloppy hack as you could literally fit what I know about submitting such things via Github on the head of a pin?
Secondary question, if the interrupt feature gets put into the Adafruit libraries, would it also make its way into the I2C display library that uses copy of those same files?