I just noticed a dungeon (technically two dungeons, same landblock) that don't list in the Dungeon Viewer. The Pyre Skeleton Cave (in-ground dungeon) and Aerfalle's Sanctum (Uber version of Aerfalle) are both in cellblock 0xB5F0.
The weird thing is that this Sanctum dungeon is "underground", below an active landblock that has surface features. Most (all? except in-ground ones) are in the water or along the border of the world.
I was able to edit the source to force this dungeon to load, but it makes me wonder... Are there others like this? Maybe even some dungeons we don't know even know about (e.g. not finished, or not hooked into the world). Something to investigate!