The zener will help prevent against overvoltage (especially in relatively small amounts, such as 2.2V) but no, it's generally not a good idea to run it with a higher voltage than it's designed for. Pretty much any regulator circuit dissipates extra power as heat... so relying on the regulator is probably not the best plan.
The Chumby Classic was designed to "[run] off 1A at anywhere between 6.5 VDC and 14.5 VDC"; conversely, the C1 "requires +5V, 2A DC power, regulated to better than +/-5%". Since the C8 seems to be more like the C1 in terms of hardware evolution I'd say it is indeed designed for 5V ± about 0.25V. Again, it's probably got some degree of built-in overvoltage protection, but your main saving grace is probably that it only ran on that for ~2 hours and so heat probably hadn't toasted things yet. If your air conditioning is set to a cooler temperature that will only help the heat dissipation
TL;DR: Chumbies/Infocasts (other than CC) are not designed for excessive voltage but have some built-in protection that probably saved your bacon for the two hours it was running on the higher voltage. Not an experiment I would recommend recreating.