We have a theoretical way to filter POIs on the client, but due to the way the new map window is constructed this means we would need to attach UI code to every single POI to make that work. Since you noticed the number of POIs we have that would blow up the POI files in size by a lot and we did not really want to do that now that SOE provided one way to filter POIs.
While the variant you propose with filtering on the server would avoid that issue (and I guess bandwidth issues could be reduced by having the updater cache all files and add/delete them dynamically to the UI folder as requested without needing to redownload), but as lordebon mentioned, since SOE said they intended to provide a way to link POIs to quests, we are kinda waiting for how this is going to be done (probably something like a questID), as it seems to be the superior solution.
You can find our discussions on this in the team forum btw, you should still have access.
