Lelouch hypothesized that the geass works by invading and rewiring the cerebrum. We saw in eps 4 and 10 how he uses it to tell people to ignore things. Now it's not that they don't see him; the geass activates so that their brains comply with the order by not caring that he is there or disregarding his presence. They have been brainwashed basically, so that anytime they see him, they don't notice.
That theoretically should allow for open-ended orders. The geass enforcement activation could just be something along the lines as "whenever you hear a command from me (my voice perhaps), you must obey" all you need is a condition for activation, like for wall-girl and
All bets are off until we find out about Shirley though.
I don't want Lelouch to just go and geass everybody into obedient slaves, but honestly I'd been hoping for more interesting and cunning usage of the Geass to achieve things, but that sadly seems to not be the case for the 1st season at least. He uses it like what once per episode max now? They mostly consist of memory wipes, ignoring things and small tasks (take this walkie talkie here), nothing big and grandiose. I want to see him get creative.