Wow. I think this is REALLY splitting hairs. Superman IS Clark Kent and vice versa. Just because he doesn't change into costume doesn't make him any less Superman. As for Mia with Quiver, you'd have to go back and change listings on a whole slew of "origin" stories to say that the first appearance of one wasn't the first appearance of the other (the Spectre, the Sword, Wraithborn--to name but a few from a wide array of publishers and eras--don't really feature the character-in-costume or the character by their super-name). That they don't appear as one or the other is really something you can and should give in the synopses or notes, as far as I'm concerned.
Not to mention that splitting the characters into two separate entries is a philosophical slap in the face to the whole concept of what it means to be a superhero.
For me, the only time I'd consider listing an identiy and a superhero separate is when the character is primarily a non-superhero, as with Insect Queen, the pre-Crisis Flamebird (I). or the like.