Hey,
I wonder if there are such lcd displays for sale that would be made of only one big pixel - when the current runs through it it becomes all black, when no current - it's transparent.
What I'm thinking about is trying to make a homemade 3D shutter glasses
The blinking keychains are almost certainly photo transparencies being lit by LEDs embedded in the plastic.
I believe that the thing you're looking for is called an "LCD shutter". I remember getting one on the surplus market many years ago, and tinkering around with driving it using an L293 (successfully, as I recall).
You can try alltronics.com, but it was so long ago that I found it in the retail store (which Dennis has long since closed to go purely mail-order), so it's highly unlikely they're still available there. But there may be some sitting on a surplus shelf somewhere else.