Hello Jeremy,
I'd be inclined to use a database behind the scenes to manage things like zoom level, location, names (searchable), and linkages between maps. I suppose you could use an XML data file or something more lightweight depending upon the size and detail of the world involved.

As ChickPea mentioned, you'd need vector graphics to keep detail if it were all one mega-map. I think the "one map to rule them all" option is the more difficult one though. I checked out the Guildworld map and it is pretty slick for what it does (zoom & pan). Kudos to the Guildworlders who set that up.

Considering that you will be obtaining maps "as they come", keeping everything in one massive vector map will be pretty inconvenient if not unworkable. If you just have placeholders for content until it becomes available then you are not forced to deal with everything at once. You can put scratch maps, rough sketches, in place of final polished maps until such time as the final versions become available.

I'll pass on participating in the data gathering. I'd be of little use anyway since I don't have anything to do with Golarion (I like PathFinder as a system but have no need for yet another world). I do wonder whether Paizo will allow such a compilation to be made available online even if it were community supported and free to use. They do own the IP rights to Golarion.

I am interested in helping with the programming side of things. Especially how it might be made more generic to cater for a wider gamut of worlds and campaign settings.
This could be basically an image driven drill-down web app. I think the difficult part will be setting up the imagemaps to drill down to lower levels. HTML can do that with imagemaps but setting them up can be tedious and if you're going to allow users to customise things then it can be complicated, but not impossible (http://iakob.com/canvas-area-draw/demo.html).
You'd probably need some "admin tools", where you could upload a map, define the imagemap zones, then put in links to other maps (at different zoom levels).
Best wishes, niekell.