Here's two possible solutions I may try.. may work for you
http://www.bpmi.nl/blog/?p=1637
From the link above, the second option given:
"Provide the Site code and Management point during client installation (/MP:<fqdn hostname> SMSSITECODE=xxx)"
A follow up for anyone still trying to work this out...The option given quoted above isNOT VALID. The interface of editing the Task Sequence on step "Setup Windows and Configuration Manager" when you enterSMSSITECODE pops a red exclamation point that when you mouse over it explicitly says, "Installation properties cannot include SMSSITECODE and SMSPROVISIONINGIMAGEMODE."
That suggestion should be disregarded and updated.
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