Since Mage Armor only gives +4 armor bonus to AC, it seems clear that it doesn't completely cover your body or your limbs.
It only covers enough of your body to give you a +4 to your AC. So it covers about as much as a chain shirt does, and a chain shirt only covers your torso and upper thighs.
Therefore, I don't believe that Mage Armor covers your limbs in a force effect, and doesn't give you any special advantage when attacking incorporeal creatures with unarmed attacks.
I could see a higher level version of the spell (i.e. Mage Plate) that totally covers your body and has the added effect of giving your unarmed attacks the [Force] descriptor, but that would be a new spell, and effectively a house rule.
It only covers enough of your body to give you a +4 to your AC. So it covers about as much as a chain shirt does, and a chain shirt only covers your torso and upper thighs.
Therefore, I don't believe that Mage Armor covers your limbs in a force effect, and doesn't give you any special advantage when attacking incorporeal creatures with unarmed attacks.
I could see a higher level version of the spell (i.e. Mage Plate) that totally covers your body and has the added effect of giving your unarmed attacks the [Force] descriptor, but that would be a new spell, and effectively a house rule.