If I am understanding the problem correctly...

Apple implemented app signing by default in 10.8. Users can either turn this off in System Preferences (removing the error), or the LibreOffice team can pay the $100 annual fee to get a properly signed app.

Actually, there's a temporary workaround which you can use and works till you update to a new version (works with any app or package not signed with an Apple ID) Simply right-click open and you'll not see the error message. No need to disable gatekeeper in System Preferences

That's correct, or users could use the right-click workaround.

Most users, however, won't know this workaround and it shouldn't be necessary to lessen the security of OS X just to use (or update) LibreOffice.

I'm sure the Document Foundation could afford the necessary app-signing.