Virtual objects occluding real objects is in fact nearly impossible. The way you're thinking about it with the extra LCD layer isn't actually a viable solution for this sort of display (or any sort of display my mind can come up with right now). The way glasses such as these work is a lcd screen hidden in the side of the glasses is viewed through a beamsplitter that combines the image of the screen with the real world. The screen appears to be "at infinity" from the viewers perspective. Unfortunately to do occlusion on glasses like this the light would have to be blocked between the source of the light (outside world) and the viewers eyes. The light blocking layer would always appear blurry from the users perspective since it would be so close to the eyes. You would also have problems aligning the light blocking layer with the screen as the apparent position of the screen in the glasses is dependent on the position of the glasses on your head.
Wow, you're totally right! Alignment problems are solvable with accurate positional tracking at least in theory, but I hadn't considered the blurriness problem. That seems like a show-stopper. It might be fixable if the light-blocking element was essentially a holographic display, capable of treating light differently depending on the incident angle, but such technology is going to be out of reach for a long, long time.
There is some nascent technology that allows the eye to converge on two focal planes simultaneously, which looks like it can solve the bluriness problem[1], but I don't get the impression that meta is using this. And it doesn't solve the issue of ensuring parallax alignment between the display screen and the background, which can only really be done if you're tracking the pupil and compensating accordingly...
