In my recent playthrough I nearly filled the memorial on the Normandy, but there are still two empty slots at the bottom! This makes me think that I can fill the wall.
Jack died in the Cerberus headquarters, I believe she shows up (alias Jacqueline Nought) if she dies under other circumstances, so that accounts for one slot.
Zaeed is missing from the wall, even though he died at the refinery. Will his name show up if he dies under different circumstances? From what I understand, this is a bug/oversight, although I've heard an alternative explanation.
Is there a character I missed who can take up one of the slots?
I know that Anderson and Shepard don't count, because their names go in the center of the wall.