Hi,
you can try implementing WOW64 into your Windows PE , I know it's a tough part, but there is a Project called Win 7 PE SE http://theoven.org/i...php?topic=131.0 which seems to have WOW64 support. I have never done sth. with that project, but I guess they use scripts to generate and copy files and offline registry settings, so there must be a script file for WOW64 (and in there you will maybe find the files and registry entries you need for your x64 WinPE to get WOW64 support).
best luck
dicht