I'm not 100% sure, as I don't use that plugin myself, but I believe Weapon Unleash is coded in such a way as that all other plugins think the new skill is the attack skill, so it should properly do atk calculation.
Keep in mind, though, that dual wielding unleashed weapons does not work correctly. The offhand weapon's unleash skill is ignored, so the mainhand skill is used twice. This is because the Weapon Unleash plugin doesn't include dual wielding support at all.