I answered my own question. I had to convert the binary image with a bit of a hack, to color.
Even though the threholded binary image just has two values for each pixel, 0 and 255, I was able to use:
binOut=c2.cvtColor(binImage, cv2.COLOR_GRAY2BGR)
The resulting image can then be set with
outframe.sendCvBGR(binOut)