Can you unset this option in the 'post install' section of the system package? (find the reg key that governs this)
but when you reboot, and before you have the chance to autologin, won't it pickup the GPO again ?
Seems to be a few options which might work.. but just a matter of testing to see when each get applied, before or after its joined the domain. I'm pretty sure the post install jobs come after its joined the domian during the run one which calls the blwrapper.