The question is simple: What happens?
There is a strong chance this happens:
On 15 November 2007, an Airbus A340-600 due to be delivered to Etihad Airways crashed during ground engine test at Airbus' facilities in Toulouse Blagnac International Airport. The brand new US$250 million aircraft, damaged beyond repair. was written off
And there is a stronger chance that nothing bad will happen, but when you exceed the limits clearly stated by the manufacturer of the equipment without any good reason then your insurance cover flies away faster than the exhaust air so if Airbus tells you to not exceed 75% N1 then it is safe to assume that it is not safe to go 76% N1 on both engines with Parking brakes set.