That's why I said I'm not sure.
As far as I know, priests don't announce who they are saying Mass for anyhow, so there wouldn't be any issue of scandal, etc.
And it's true that Mass can be offered privately for any soul.
I've never come across that question before, but the more I think about it, the more I say it would be fine.
Though it's true that charity should extend to those of the household of the Faith, and family members, before those in the general public at large. That's just a general principle though -- it certainly doesn't mean we can't show charity to a random stranger. (Though it might very well mean those in your family are already taken care of. If you had a sister who was starving, you'd do well to take care of her before a stranger who was just as starving -- if you had to choose, of course)
Matthew