Okay, sounds like your subzone bitmap data is laid out in exactly the way that I would have recommended. The only way you might be able to squeeze more performance out of this system is if you can be smart enough to prefetch things in advance of when you actually need them. That would require all manner of UI integration. "Ahh, he's scrolling north; prefetch the blocks!" Alternately, you could look at compression of various sorts.

Still, it doesn't look like image data performance is really your problem. Instead, your problem seems to be all about managing your metadata efficiently, so you can quickly go from a (zone/subzone,x,y,z) tuple to the actual image tile file and offset. That sounds like a job for a database of some sort. You (apparently) don't need a full-blown relational database, but you may find something like Berkeley DB to be a useful in-between, particularly when other tools like Perl play very nicely with it, which would help you to build the database in the first place.