Is it legal to fly GA aircraft in temperatures above what is shown on the performance charts?
Yes, it is legal, disregarding any catch-alls such as 91.13 reckless operation or ones pertaining to pre-flight planning data like takeoff distance.

It is simply left to the pilot's good judgement for a go/no go decision depending on whether or not the flight can be completed safely.

Edit: Re-reading this answer much later, I would like to emphasize, "disregarding any catch-alls".

With regard to the catch-alls such as 91.13, this could very easily be considered illegal, but no rule with a statement clearly identifying taking off with temperatures higher than those on the charts as a prohibited action exists to my knowledge.  Do not do it.