I probably phrased it wrong. I understand about the plan for composite onto live plates, I was thinking more like for example cgi slime coming off of a character and being blended into real slime in the live action shot with masking and feathering, just hybriding to make if harder to tell where the sfx end and the reality begins. People do this for example with fog all the time, use the real life fog machine in the shot, then art direct with alpha channel animated fog, so you can get both the authentic look of the real fog, and the post flexibility of the artificial fog.
It was a popular and effective technique back in the old days, when CGI was prohibitively expensive and every frame or even partial frame was costing the studios a fortune. It looks great when you mix them, as long as the audience can't tell how or where you mixed them. Little bit of real fire in focus, and a lot of cg fire in the bokeh, that kind of thing.