To be fair, I think Julie was suggesting something that worked alongside the original rather than completely replacing the electronics
Hence the bit about being in time with the PETs pixel clock.
Strangely enough in my youth, inspired by the graphics add-on for the Research Machines 380z, I dreamed up a scheme to make a pixel-graphics mod for the PET that would overlay the text display by using the internal logic timing but connecting to 8k of RAM. It never got beyond the (graph) paper-and-pencil stage because I went off to uni and started playing with "big" computers, but I recon it would have been technically practical, even though it might not have been financially practical. Also things like the Acorn Atom did it better in colour and cheaper by then!