As I think I mentioned previously, I was using a X/Y line driver module to drive the inhibit lines - forgetting that since only one H-bridge could be active at a time, at most only a single '0' could be present when writing a byte to memory. Some hasty adaption of the X/Y line driver, and we have the Inhibit Line Driver, Rev A. The PCB arrived today:

Now I'm just waiting on some 2512 SMD resistors...