If you are entering it as you said, starting with capital letters, there is no reason it should be rejected, and the most likely cause is that the script can be a bit buggy and has been known to reject handles that are too similar to an existing handle. You can try modifying the first or last name a bit and it may accept it that way.
The dev team leader is currently on vacation for a few days but I will point him this way when he returns; perhaps he can do some tweaking to loosen the restrictions a bit.
I ended up using a different name, 'Cortez Lecato', for the signup. But I'm wondering as to why my first choice wasn't accepted. It's not really important to me, I'm just glad I was finally able to join. Still, it's interesting.