This would be super useful for INTERNAL processing, to generate certain effects by checking the pixel values. For example:
An image that is a map of how fast you can move around, or how much environmental damage you take. (Need not be full 8 bits.)
An image that is used to generate custom dithers or wipe/transition effects.
An image that is an internal world map, from which tilemaps are generated as needed.
(Grayscale PNGs might also be displayed directly, using a chosen dither. Dithered images can be prepared externally of course, but doing the dither in-app could be nice for making the dither closely match other game content.)
(Meanwhile, although direct Lua support would be awesome, I'm looking at workarounds like multiple 1-bit PNGs, big numeric tables, and a nifty C library RPDev posted.)