This hot patch does what I want... immediate send instead of accumulation. Not sure I can recommend it for general inclusion since it raises the spectre of how to handle an error that happens after some of the body has already been sent.
sed -i "$(fgrep -n "result_accumulator['stream'].append((msg['content']['text']))" /usr/local/lib/python3.8/dist-packages/kernel_gateway/notebook_http/handlers.py | cut -d: -f1)s/[^ ].*/self.write(msg['content']['text']); self.flush()/" /usr/local/lib/python3.8/dist-packages/kernel_gateway/notebook_http/handlers.py