Re: potential alternative power?
What he's saying is that he wants to use the exhaust from the engine as a source of compressed air to drive the pneumatic wrench. Not another engine, but the existing engine that already drives the rear wheel, then use a pneumatic wrench to assist with taking off.
You might be able to get metal seals made, but once again I wouldn't see it lasting very long based on the heat.
Another thought is how much pressure are you expecting the engine's exhaust to pump out? Because at a certain amount of back pressure, the engine will just cut off.