There is a reason diesel engines have such great longevity, the diesel fuel acts like a lubricant of all the systems it touches. Petrol it a fantastic solvent (I clean paintbrushes with it if I don't have any white spirit or kero) and will chemically clean the injection pump, injectors and cylinder walls of any lubrication left by the diesel.
Have you ever had an injection pump in pieces? Metal components in a metal housing, a metal piston fitted so precisely into the bore that it can inject diesel at 280Bar in the 300tdi, you can't actually rebuild an injection pump without wetting it with diesel or a very light lube first. A dry injection piston will not go into the bore without making horrible metal scratching noises if you can even push it home anyway, a little diesel and the thing will slide in perfectly - imagine what would happen in the presence of a lot of petrol and this thing is moving back and forwards thousands of times a minute.
The petrol in the mix will change the dynamics of the diesel burn and will burn differently which can lead to heat issues, melting and piston damage.
Diesel injection also relies on a shock-wave to fire the injector, it's not just high pressure behind the pin that causes it to lift and inject the fuel, it gets shocked and it needs a heavy enough oil to let the shock-wave properly travel up the oil and get the pin to pop up to give a good clean injection, poor injection can cause injector tip and piston burn - also the fuel you intend on using is of a higher octane which will lead to greater issues.
Why don't you sell the Avgas to someone who can use it and buy diesel.