I had always suspected it was by that designer brand. The cut definitely fits their style.
The color appears to be a bit darker than the one Charisma Carpenter wears in the episode, but it could be due to room lighting or other circumstances. But the cut and everything else appears to be an exact match! I won't have a better idea, until it arrives.
I'm happy to finally find this iconic dress. It's one of my all-time favorite Cordelia Chase fashion pieces that screams Queen C.