barit1:
In a gas turbine engine, there is a strong thermodynamic case to be made for mid-stage cooling of the compressor airflow. It would not be a pretty picture, with additional air ducts or fuel lines running hither and thither, but it would be a useful way to heat fuel in the tank and simultaneously optimize the engine cycle.
Please don't even think about ducting bleed air out of the engine. I had to investigate an accident where three of my colleagues died because of a bleed air leak. Yes, it was decades ago, and yes materials have improved, but bleed air is highly dangerous stuff. Introducing bleed air into a fuel tank frightens me and I'm fearless.