If you put the string NSNavPanelExpandedStateForSaveMode with a value of 1 into /Library -> .GlobalPreferences.plist or ~/Library -> .GlobalPreferences.plist, these sheets will be expanded by default. Keep in mind that this is a per-application thing, so if you have already opened an app and used the File -> Save (As) mnenus, you will still have the sheet how you last left it (expanded/collapsed and size).
You can use this Terminal command to set this value:
defaults write /Library/Preferences/.GlobalPreferences \
NSNavPanelExpandedStateForSaveMode -string 1
I imagine that you could manage this pref for your clients, but I haven't tested that (I just set this on our images).

