Connecting external displays over HDMI to 2016 MBP Touch Bar requires a restart
I found a workaround for my setup with HDMI-USB-C-conversion: first plug the adapter to the computer, then the display cable to the adapter. Then the display works immediately.

If I plugged the monitor first into the adapter, it already "gives a signal" by showing a black screen instead of the "no signal" message. If I then connect the adapter to the computer with the monitor cable plugged into the adapter already, the display would detect, but the black screen would remain on the external display.

Not perfect for workstation use, but for meeting room situations, it's acceptable.