The problem with this has been mentioned several times. It's not a lack of doors or team time that causes the "housing shortage" - it's server space. A NWN Server can only run so many areas simultaneously in a module before certain issues start to crop up. Layo, if you haven't noticed, is huge.
As a result, adding even a single area is a matter of huge debate (the pros of the area vs. the cons to the server) - and housing is lower priority than new plot-related or world-building areas.
So while it's a nice idea, and has been proposed several times already, it isn't a practical solution. There have been rumblings that the Team is working on something, however, so don't give up hope for affordable housing for the masses.
Edit: This post illustrates an idea in a similar vein - and its problems.