(imported comment written by SystemAdmin)
This is the type of actionscript we use in our patch content, so maybe it might give you some clues (we usually suppress the dialog box and the reboot option if possible):
waithidden __Download\office2003-KB2493523-FullFile-ENU.exe /q:a /r:n /c:“msiexec /p MFC11.msp REBOOT=ReallySuppress /qn”
One thing you might be missing is that you need to manually call “msiexec” to run MSP files.