hey, thanks for the tips! another great method for dithering pictures with custom patterns is dan fessler’s index painting - if you make your gradient map to match your palette and posterizing steps. that method works even in after effects.
however, i just found out that the script isn’t actually necessary 
you can select installed dither patterns when converting to indexed (sprite -> color mode -> more options)
patterns seem to be duplicated (ordered dithering + name, old dithering + name (for some reason this seems to get better results)) but they work.
i can’t believe i didn’t notice it sooner.