When is it okay to update other search committees?
Don't make assumptions. Nearly everywhere will let you know when you are no longer under consideration. You may or may not be high on their list. You could be high and they are just dithering. But they would rather make it definite, just to avoid fruitless communication. 

But it is probably a mistake to tell them you've had an interview, as it may confuse them about your interest. Wait until you have an acceptable offer before you notify them. 

Trying to "nudge" them in your favor could work either way, if it has any effect at all. It is a risky thing to do. Nudging might be worthwhile if you are juggling multiple offers.