It looks like the way I used an AVR to take over the Z80s bus is fairly succesful, and could be used to Internet-enable more old-school devices. For this machine, the persistent highscores are also a nice improvement.
If I were to do this again, I'd probably redesign the circuit a bit: the latches meant to give the Z80 access to the address bus are connected to the data bus too. That adds a bit of load; every device controlling the bus needs to output a bit more current because of it. They can be replaced with shift registers, which have the extra advantage that they wouldn't need as much wiring to control them.
As usual: the sources for the firmware is downloadable and licensed under the GPLv3, the same license the Ethersex framework uses. I've included the git metadata so you should be able to pull in upstream changes to the Ethersex framework with a simple 'git pull'.