Now that I think about it...
Who did conceal Lelouch's Geass for that year when his memories were rewritten?

This is to be discovered, since he had no special lens and his Geass was still active when he was brought to the emperor by Suzaku... so I doubt CC was able to stop the Geass later.

That could mean the emperor can rewrite memories and conceal/rewrite what his necessary for the Geass to express itself. The emperor might be the strongest Geass user, or possesses someone/something who has extended power/control on anyone's Geass.