Have you used flash with Live Composite?
I did but my setting was not sufficient. From AI claimed that the flash would fire on the initial shutter press but it fired on both the first and second press. But I can't find the image.
My purpose was to use low power flash 1/8 or lower to have the base image exposed sufficient to have the background bright enough to have detail but not so bright that is took away from the night time.
I'll be tiring it again and update in a comment.
Have you attempted and what was your process and results?