I luv been about to stream my music to every speaker, I know you can using line-out you can setup up headphones, but it's fair from ideal, so I suggesting something altogether 'simplier' - pick up your headphones, select what you want to listen to on that 'zone', (play it loud - optional ) and not disturb anyone else in the house.
I know by saying "simplier", this by definition means it'll be harder to implement. But hay look at what the Sonos team did the first time they tried something 'never done before'. :))
What do you think?