Could hydrazine be used as a scramjet propellant?
Technically yes, you could use hydrazine in a scramjet.

But there's no reason to do so. The singular purpose of building a scramjet is to get air into the combustion chamber, so that you don't have to use a monopropellant, which all have high cost and poor energy density. 

Kerosene or H2 won't burn without air, so you need a scramjet or an oxygen tank to burn them. Hydrazine doesn't, so you can burn it in a rocket, at higher pressure with more efficiency and better aerodynamics.

Advantage: Rocket engines have a thrust:weight ratio of up to 133:1, scramjets about 2:1. So an equivalent rocket is up to 66 times lighter than a scramjet.

GE's patent refers to the use of a small amount of hydrazine in a catalyst-like role, or more accurately to act as a fuel/air mixer.