I was just about to post virtually the same problem.
I was successful in creating an Admin Install Point with
Office 2003, created .mst files AND successfully deployed
it using Group Policy (No users were prompted for the
install key by the way; it was in the .mst files, but it
is also sitting there in the .inf file on my installation
I used the following syntax -
msiexec.exe /p <PATH>\MAINSP1ff.msp /a
<PATH>\pro11.msi /qb /lv* f:\SP1.log
which is essentially the same thing that you did.
The exact message I got was "A valid Product Key must be
set to create an Admin image."
I can't believe I wasted this much time. I should have
checked out the NG first.