So... this is a big question, is it fundamentally possible for engine to support more than six players simultaneously? Will the engine allow for multiple instances of turn based combat running at the same time within different areas? Speaking of, does the engine even allow to have players in different macro-areas (the ones that have a loading screen when you enter them) at the same time?

These are all fundamental questions, really, I have no illusion of trying to achieve this with the current set of tools, but the potential intrigues me.

Wow... NWN was my best experience in online RPG ever...

Simply 4 Player by Tiaexz, is an attempt to this way.

I've not started to mod this game yet but It may require the direct intervention from the developers more than simple mods, because it seems needs to be changed the way that the engine works. NWN supports dozens of players at same time, persistent worlds are not story-driven like this game, there is a Dungeon Master, and the PvP requires being in different parties. And also, there is the fact that you already have NWN to play like this, there are a lot of persistent worlds working today if you want and its graphics are not so outdated till today.

Just as a small correction, I haven't been working on a persistent world mod. I have only been working on expanding the customisation process and the linked mod is simply an 'enabler' using the games current constraints to simply allow 3 and 4 to control a henchman. I am trying to deal with an 'extended' which allows multiple customisation of 4 source-hunters at the same time.
Thread here: http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=520328

Whilst multiple instances of turned combat might be already in (I would need to boot up the game to check..), there is no current macro-area at the same time. As for persistant world, it would require the ability to host a server. This is something I don't believe is currently supported or implemented.