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