I think you probably won't have any trouble if the site-wide-admins are carefull to not write back your changes in words of the message-keys back to the message catalogs and just keep it into the DB.
So, you change all the words / sentences that you might like, but you don't re-write the catalogs with the info in the DB. So no changes are seen in the catalog files but the words for the user will be changed. The only thing is that you should do this changes in the live site, since there is no way to pass the word message-keys changes done in the DB from a dev to prod, unless you dump your keys.
Will be easier to do it into the prod using the UI from acs-lang.
