I like the idea of a special graphic for hidden nationality units. That will be a welcome change in my book.
On the subject of hidden nationality units not being so hidden, it would also be nice to not be able to see the promotions on hidden nationality units. I mean let's face it, a hidden nationality unit with a race promotion is a dead giveaway. It wouldn't change anything in a single player game, but it would make a world of difference in multi-player.