I am new to the thread and have read more of the first posts and skimmed towards the end but if you want to do something your way use a microcontroller. It is too bad that the the Mega328 is limited to 16 digital outputs plus serial communication. Two more outputs would drive two alphanumeric displays. Possibly the SEVSEG library can be used as you originally proposed with a lot of bit manipulation to program the characters.
Can you imagine a bank of stand alone ATMEGA328s or ProMinis having more than eight I2C addresses? Maybe there is a standalone version of the Teensy ++?