Interesting question. My guess (and it really is a guess) is that Google's algorithm will have no way of knowing, let alone taking into account, that it's a "masked forward" and will look at it for
SEO purposes exactly like any other domain-name with a site on it.
Looking forward to seeing other answers and whether I'm wrong about that (partly because I have exactly that arrangement myself, for my own blog - not that mine is keyword-researched or anything).