I believe Epson's inorganic grayscale LCDs used in their 3LCD projectors has decent aperture ratio / transmissivity so it may indeed be possible to do mega contrast by sandwiching 3 more LCDs next to the original ones.
I'd love to try this but 1080p 3LCDs are still quite expensive.
However dual gray LCD is probably the simplest design for mega contrast for DIY. I have a DLP I will take apart when I buy a new projector to try and rid up dual modulation somehow (either dual DLP or DLP + grey LCD) using spare parts from ebay.
In theory the back panel only needs to be low res, but high Tx ratio. Giving up 20% of the light is one thing, but 20% from 0.5 is 0.4, which is quite poor. Although doubling lumens is do-able on DIY. At least for SDR. High lumen single chip LEDs are quite cheap these days.