I'm finally ready to release this update to the Dexter firmware! To test Dragon's Lair 2, I modified Daphne to communicate with a real serial port (instead of an emulated one). I then had the real serial port connected to the DB25 port on the Dexter board (as pictured). Note that I am correctly using the null modem adapter!
So Daphne is emulating Dragon's Lair 2 (except for the laserdisc player) and Dexter is emulating the laserdisc player. It is working except for picture stop codes being ignored (which is currently unimplemented in Dexter but I guess I need to get wookin' on that!).
This will still need to be tested on real Dragon's Lair 2 hardware!