Quote - Render once with hair and hat, and once with hair invisible (just an area render will do)
Photoshop (clone) the hat over the protruding bits of hair.
For god's sake, magnets?? custom transparency maps??? Has everyone gone mad?

 

This is fine if you are just doing a still....if  you happen to render animations like I do then magnets are the only way to go IMO. I find it usually takes a minumum of 2 and usually 3 or 4 magnets to hide the hair under a hat.

Ed