So I went through the 2004 regulations for the LMP1 and LMP2 classes and found that you can indeed place the air intake for the engine in the 2nd headrest that is mandatory for these classes per the 2004 regs... why courage decided to create so much more drag and put the JPX air intakes on top of the driver's headrest is beyond me, maybe they just didn't see this possibility. However, it seems that Lola has thought of this as it is shown in the concept sketch of the new LMP2 car which you can see here:
http://www.mulsannescorner.com/newsnov03.html
it seems that the drag costs of the new 2004 regs on open top cars won't be as bad as though if car makers go this route...