Well, good luck, just remember that worldsave size won't stay the same all the time.
As game grows (both in content and playerbase), so will worldsave; each and every time world is saved, engine will have to waste time on compression, while in reality you'll need only very small % of nicely compressed files - when you restart server. Just take notes how many files are created, and how many are actually used.
I'd rather work on external tool/script which takes care of compressing worldsaves once engine creates them; just making sure N most recent files are left as-is, so they can be loaded after restarts. Having tool/scripts is simply easier to maintain in the long run - if you ever decide to update engine, you'll still have desired functionality, without need for porting engine changes. Same if you decide to work with team which, for any reason, cannot implement your changes. As a side-bonus, it could be adjusted to work with any other application generating files constantly, not just FOnline server.
Just my two caps
