Do you know if SP2 is distributed as an *.msp? You can check by running the SP2 install on a system and monitoring the %TEMP% folder to see if an *.msp gets laid down.
If it's a windows installer patch, it opens the option of creating an Administrative Image, and then applying the patch via:
msiexec.exe /p
/a
...But you have to be careful with this as well, since Microsoft has Best Practice issues with this just like Nested MSI's. Really, the best option would be some sort of bootstrapper that calls the main MSI and then the SP2...but I'm only aware of the MultiPlatform suite installer that will do this out of the box.