I think I'm answering my own question but I'd appreciate a confirmation from those 'in the know' of possible.
Looking at the ADV7604 support files, it appears that it supports RGB/Component inputs only. It has no ability to support NTSC/PAL baseband signaling, sampling and conversion - which the ADV7842 happily does.
Is this correct? Am I completely out of luck trying to make a ADV7604 encode CVBS?
Thx in advance.