Still a bit busy over here but I have sporadically been developing the ZAViouR idea. I have now laid out the main board after adding a soft reset switch to allow soft resetting of the Z80 via the AVR and another LED for general use as a status indicator:

The 8-bit I/O Port:

And the Backplane:

The I/O port will plug into either the main board directly, or into the backplane. The backplane has provision for daisy-chaining more than one board together in case four ports isn't enough.

When finances and time allow, I will be ordering these boards to build and test before publishing all the design files on www.projectavr.com