Righto; in having a think about this last night, I realized that the NTLITE unattended menu is basically just 1:1 with the capabilities of sysprep, and I do not believe that the user creation functions of SYSPREP/ADK/autounattend.xml support passing that flag to the newly created user, so if that is true, then one way or another, one is accomplishing this after-the-fact.
It would just be nice to bury the function inside the auto unattended menu.