Using vertex color in post processing?

From what I can see using the material editor, vertex color is not an available option in scene texture nodes. I’m assuming that means vertex color is not available in the g-buffer.

Is there a way to add vertex color to the g-buffer, or somehow pass vertex color to post processing, as a scene texture or in some other manner, without using channels like base Color?

This is possible, but you have to modify the engine source for the GBuffer to get vertex color info. This guy did it here, but didn’t give any info on how he did it (other than saying he used the alpha channel to pass it):

(slide 30)
https://www.slideshare.net/TomohiroOgiwara/ue4meetup-63696087