"Mixing" output signal from the µPD6451 with "live" video is not difficult, if you go for white characters on top of everything ..
Program it for WHITE colour and signal will be available at RGB pins and Vblk ..
Of course, you need to extract V and H from the video signal, and that can be done with something like GS4881, or similar ..
If, however, characters have to be displayed in colours, that is a different story, and you will need to employ some sort of video processor with RGB inputs ..
Regards,
IanP