I'm pleased to report that upon fitting the last two chips and uploading some Z80 'Hello World' flash code, the board works well. Programming both the AVR and the Z80 require the /BUSRQ line to be grounded with the manual switch but apart from that, it seems to operate as expected. I have only addressed a single line on the output port so I'll have to write a bit of code to access several to test it properly.

Now I'm thinking about the new version which will sort out the /BUSRQ problem and add a couple of features. Requests considered.