Would it be possible to first model the object, render it with yafray, save it as a .blend file, and then open it up in blender again and composite it into live video (which would then be rendered with blender’s internal renderer). I was also wondering, what is the most photo-realistic renderer supported (or one that can be integrated into) blender?
Yoiu can render anything and composite later once rendered. The most realistic renderer will depend on what you render as every engine will have it’s strengths. Yafray is the only external renderer that is integrated,
Hoshi is right, Composite nodes can be used with Yafray, material nodes not. Also, it is true that only Yafray is really integrated into Blender but: there are (non-official) builds around which integrate Kerkythea into Blender and a lot of Scripts for other engines which work well. Good enough at least.