I raised this at the Ready Tour when I presented on Windows Deployment Services but I just wanted to discuss it again.
Theres been conflicting information presented around whether hotfixes and service packs can be inserted into a WIM image thats been mounted up. The basic guidance is that if you can open the hotfix such as with the process below then it works. If the service pack, application or hotfix is in an MSI format or an executable then it cant and must be installed "online" which equates to an installed running Vista OS.
Process: Once the image has been mounted RW or applied, you need to expand the hotfix package to a temp folder such as expand qfe.msu –f:* c:\temp, then use pkgmgr to apply (start /w pkgmgr /n:c:\temp\qfe.xml /o:”c:\;c:\windows” /s:c:\sandbox). You can check if it is successfully by using echo %errorlevel% command. If return value is 0, then it’s good.
Credits for the process: John Liu (OEM Engineering).