If all you need is to redisplay what's on the LCD on board A, then the MCU on board B may be redundant. The fact that board A is able to write to its LCD ensures that it has, by reading the busy flag or by open-loop timing, determined that it is appropriate to do that. The result is that you can rely on the relative timing to be adequate. If you involve a second MCU, then all bets are off, at least if the second MCU does its own retiming of the interface. Board C is just the LCD module and the serial interface, right? If there's no other need for the MCU on board B, I'd guess you would design it in but not populate the MCU site. There's no need for the MCU just for appearances, is there? If all you're doing is redisplaying what's on the LCD module on board A, all that's needed is the CPLD and a cable connector.

RE