For PXE booting Windows PE requires Microsoft Remote Installation Services (RIS). RIS has got speciali server-side code that authenticates Windows PE with the server itself for access to the network share with WinPE's system files.
I've tried and have come to the conclusion that PXE booting WinPE without RIS is not possible.
P.S. I'm hoping to be corrected of course, but I'm not holding my breath

Cheers.