Polyimide resins are employed for high-temperature applications, where the upper limit of temperatures is around 450 F.  I cannot say with 95% certainty that thermal cycling will be inconsequential, but I have a hunch that the higher melting point and lower coefficient of thermal expansion sound promising.

But I have no idea where to find the stuff.  

I've had plenty of success with roughing up aluminum with a good gritty sand paper and painting it with engine paint.  Just make sure that you prep the heck out of the part before painting.  New heads still have oils from machining on them and you're going to have to make sure none of that has soaked into the surface you're painting.

On the rough cast portion of the head, I typically hit that with a lot of brake parts cleaner to really wick the moisture/grease out of the pores and then follow with a good prep-wipe cloth to be sure it's all gone.

I've always found that the more time I spend prepping a piece, the longer it stands up to what I throw at it.