Just a very quick update since my last post.

It occured to me that I could simply use standard IRQ handling for the VIA input from the Pico (for PS2) instead of the CA/CB handshaking. This means that by dropping the handshake and moving to Interrupt handling, I now have one open pin to keep track of the 65C02 clock. Eventually, I may even decide to use the Pico to drive the clock, rather than just sense it. This again would reduce the overall chip count by one, and maybe add some flexibility. Plus, it wouldn't be a design change that makes what I do be incompatible with the standard BE6502 architecture (which is a goal for my project: I want whatever I do not be so specific to my design that nothing is re-usable by anyone else).