Not a bad idea. You could do it with one person in the theater, but you can take it one step further.

Have a camera facing the eye that can tell what the person is focusing on. (High end video cameras already have this.)

Have an adaptive lense in the glasses to vary rapidly change the focal plane from where they eye things it is to the screen. You then build software that blurs the image outside of their focal plane. Granted there are limits on this aproach, but people get used to glasses that have a fixed shift in the focal plain fairly quickly so it should be possible.

Now I don't see how you could extrapilate that to a theater full of people, but you can build a system that works for 1 person.