While the reaction of the common 1602 module to commands is indeed in the tens-of-microseconds-and-slower range, the hardware interface (pulse lengths and mutial timing of the signals) is in the tens-of-nanoseconds range, which almost completely excludes a purely software solution.

I would use a latch or two ('574 + '74), simply latch the data plus the RS and RW at the falling edge of E, and feed the latched RW to an external interrupt set to edge sensitivity, inverted if needed, to have the interrupt triggered upon writes to the LCD. The ISR then would need to reset the RW latch, in case the original device does not perform reads from the LCD.

Not a bad idea, this is. I could find quite a lot of uses of it.

JW

PS. Anybody fancy a common 8052.com project?