You're seeing things. I've never once noticed Leviathan use one of my previous Shepards, not even after playing through the DLC with five Shepards.
Well, at least one of the faces I saw during the DLC reminded me a great deal of my previous Shepard. This is however probably just a coincidence. Bioware just used a ordinary face editor to build facial structure etc for the character instead of doing a more detailed model. — Dogmatix