I bought this chip off of Ebay. It seems that Zilog created a uPD7220 clone - and it works! These chips seem to be rather rare and information on them is scarce. I did manage to deduce that there were two variants of chips: one operating at 6Mhz, and one at 8Mhz. I got the 8Mhz one and tried doubling my pixel clock to 16Mhz (the GDC uses a halved clock). That worked too, and that allowed me to make a more stable video signal. I had a problem with the 8Mhz pixel clock where there was not time to output a valid video signal. Now I can output more pixels than I have VRAM for.
The chip is also a super pretty Zilog gem <3