Just to summarise but your "Third Hand Belt" is essentially a belt with the stats of a shield right? In that case isn't it simpler to make a belt with shield like stats than implement an extra feature when in practical terms there isn't a spot in playerdoll and probably code needs changed too to accommodate it.

Personally if modifying equipment slots I'd say that following ideas should probably be implemented first
1. Ability to equip two 1h weapons and go without a shield (i.e. dual wield, currently I think there are issues with timer)
2. Ability to equip two shields and go without a weapon (e.g. for priests or mages who use magic etc)

I'll also note that some of equipment is designed to make certain class/guilds more effective. That means that when designing a character to have certain advantages you need corresponding drawbacks for game balance. Being unable to use 2h weapons is one of them for mage/priest guild. Admittedly _person_ has a combat balancing thread so it's a work in progress

I think that mages with 2 shields wouldn't be good, i mean how do you cast spells if you have 2 shields, maybe add some kind of staffs into the game that increase magic damage or resistances, Staff of protection, staff of destruction and so on.
About dual wielding, i was thinking about that too. It is cool, and would be nice to have.
But only after it is balanced, i mean 2h weapon has a chance to fumble, but with 2 swords that don't even match the amount of dps of one good 2h sword would fumble too much. Also you'd need to have a lot of good 1h weapons.
What i am saying is it all depends on how dual wielding is coded, i mean if it works like 2 different attacks, if it is one it is ok. But dual wielding to make it more realistic should have 2 attacks, with a small delay between them.