Hi Mike,
I have not tested this with x64 UEFI. I'm not aware of the changes needed on the PXE server (Could you explain this, cause I would like to test x64 UEFI network boots).
What version of LANDesk do you use? I suspect this to be a typical "bitness issue", where you call 32 bit programs in a 64 bit environment.
Googling this also directs us in this direction:
Create a bootable WinPE 5.0 x64 USB drive - Page 2 - Tutorials - reboot.pro
ImageX in Windows PE is complaining that the subsystem is not present - Server Fault
Could you please cross check the bitness of the environment and tools? Also, could you perform this step manually on the WinPE command line? This way you should be able to troubleshoot this issue better.
Maybe C:\Windows\Logs\DISM\dism.log contains some useful info.
Best regards,
Carlos