mattmiller wrote:Well _ i'll tell you all how long it lasts - jsut powered one up - its managed to boot and I can connect to it over WiFi
Thanks for trying it and we now have an answer to the actual question asked.
AIUI the "5V" goes to a regulator which powers everything else, to the USB sockets, probably the HDMI socket, and GPIO power pins. Providing USB, HDMI and "5V" rails were not used it would seem entirely safe to use any voltage which doesn't exceed the regulator's input voltage or cause excess heating.
Using a USB device at over 5V would depend on its ability to withstand over-voltage. I wouldn't like to connect it via HDMI without knowing exactly what the Zero's HDMI interface was.
If we had a full circuit diagram of the Zero people could see exactly where "5V" went and wouldn't have to ask, guess or try to trace that out, but I don't expect to see that any time soon.
I would suspect that the biggest problem will not be running above 5V but what happens as battery voltage drops, as regulators fail to maintain their output voltages.