The more I think about this the more I wonder if it might actually be possible - an R-2R ladder on the external memory pins (34-41) together with DMA should work reliably at 10.5MSPS and may even go as high as 21 or 28MSPS (I learned that doing the VGA library). The dealbreaker would be how close to the exact colour subcarrier frequency you have to get. In my experience, with old CRT TVs you have to be very accurate, but more recent LCD panels have more tolerance for the frequency and amplitude of the colour subcarrier being wrong.