Look, how is it it different to be able to make "off-hand" unarmed strikes or to be able to make MORE attacks with your available multiple appendages and take a penalty for it?
I see it that to make a main-hand unarmed strike you need to use your main hand - monks excluded.
So you could two-weapon fight with your main hand unarmed, and a dagger in your left.
You could fight with the dagger in your main hand and unarmed in your left.
That obviously means that "hands" are capable of both main-hand and off-hand fighting, and each time only using ONE appendage.
Following the logic chain a little further this obviously means you can use both your main and your off-hand as unarmed - taking the appropriate penalties.
Quoting the SRD again:
If you wield a second weapon in your off hand, you can get one extra attack per round with that weapon. You suffer a -6 penalty with your regular attack or attacks with your primary hand and a -10 penalty to the attack with your off hand when you fight this way. You can reduce these penalties in two ways:
* If your off-hand weapon is light, the penalties are reduced by 2 each. (An unarmed strike is always considered light.)
* The Two-Weapon Fighting feat lessens the primary hand penalty by 2, and the off-hand penalty by 6.
Now I'm too lazy to look into the PHB, but the SRD gives no indication as to how many unarmed strikes a character gets. Monks excluded unarmed strikes are supposed to be made with hands, I suppose. This is of course interpretation, but it figures since if it counts as a weapon then it can take the Main-hand or the Off-hand slot. So it figures that you always have as many unarmed strikes as you have hands if you take the appropriate penalties for two-weapon fighting or multiweapon fighting. The key issue here being, of course, BAB and the number of attacks you can make.
Now only for Monks is this complicated, since they have the iffy paragraphs about using all appendages and also not having an Off-hand. So monks, by strictest RAW, cannot TWF with unarmed strikes, because they cannot use the Off-hand to make unarmed strikes.
This is, of course, total bullshit, and any DM with a scrap of common sense should simply ignore the bit about not getting Off-hand attacks, as long as they DO them as off-hand attacks, meaning, in the chain of priorities for attacks:
Flurry
Main-hand iteratives,
off-hand iteratives with TWF, ITWF, GTWF, PTWF
Natural weapons at -5 or -2 with Multiattack
For monks now it clearly doesn't matter what appendages they use for either main OR off-hand attacks, as long as they take the appropriate PENALTIES that balance out the ability to do off-hand attacks. Also, any unarmed strike taken as an off-hand attack should also only do 1/2Str, since we are already ignoring the bit about "they get no off-hand".
I wouldn't even count this as a house-rule, it's simply a common sense clarification.
If you were to make a house-ruled errata it should read like this, replacing the blurb about not having an off-hand:
A monk can use any appendage to do unarmed strikes. While using her MAIN iterative attacks or Flurry of Blows a monk always adds her full Str-bonus to her damage rolls for all attacks, no matter which appendage she really used. When doing off-hand attacks in addition to her main iteratives she adds 1/2 her Str bonus to her damage rolls for those attacks.
Of course if you want to re-balance you could just still let her add full Str bonus. Now that I would call a house-rule.
Now as to taking Multiweapon fighting: This feat has a clear prerequisite: Three or more ARMS! That should clear things up. Which also clearly means this works perfectly well for Thri-keen.