Hi there,
I would agree for sure a total software solution would be the most desirable, The Bit Mask @grobasoz pointed out should work and imo would be the best, but also a Heads up on the keyboard link some user reported the pinout is incorrect? It’s a funny challenge, I thought it would be easier. 
I’m more surprised that Keypad.h doesn’t already have something like or for it.? 
GL 
PJ 