hmmmm. for the record, in my campaign world, it is impossible to forcibly mess with the moral and/or ethical agency of an individual by any means or power up to and including artifact, epic, and deity level effects or actions. i strongly disagree with the existence of such things. which has a number of odd and interesting affects on game mechanics, magical effects and items in said campaign world setting.
having mentioned that... i suppose it would depend on what you decide for the wording of the mechanics of the mask. if the mask functions by effectively (if temporarily) lobotomizing, or somehow imprisoning the reasoning/ethical/moral agency of the solar, or replacing the personality or some such; then it would be the mask that is evil and the solar retains its' good subtype, which would likely make it immune to holy word, but not the mask (if and only if it functions as if it were, or in fact actually is, a creature of some type).
if the mask is not in any way a creature, and/or functions by changing the subtype of the creature, along with the mindset or whatever... then holy word would be effective.
your use of "appear/behave" inclines me to think more the former where it does not actually change the essential nature or agency of the solar, thus holy word would not be effective, in my opinion.