I can also confirm that Defense chance will be calculated through Guard damage so long as the originating damage could've been mitigated (i.e. Melee/Ranged but not Force/Tech). You can easily prove this by dealing damage to the targeted player and see that, occasionally, damage towards the tank will not be applied at all. One caveat: This will not show up on a parser, and you will not see text indicating that you have dodged a guarded attack.
Again, great work! If you need help testing and you are on Bastion or Harbinger, let me know. I'd love to help.
I think my guardian tank has a significant resistance percentage, so unless the attacker (e.g. sage) is stacking accuracy, some of his attacks should miss me.