Why *do* they need to be POWDER_MISC_CREATURE? Admittedly creature and plant mods are more my forte, but the raws for gypsum plaster don't refer to a specific kind of powder at all, and that'd be the most similar thing.
Because I wanted them to be stockpiled in some way - plaster is listed as [IS_STONE] and then [NO_STONE_STOCKPILE]. It's not on the stockpile menu at all. You have to use a stockpile for the
bags to get the bags with gypsum powder in it where you want them, and even then, it doesn't discriminate between bags that are empty or filled with gypsum powder.
Don't ask me why, ask Toady. I don't know why he didn't make a list for stone powders, especially since there are stone powders in the game, but there aren't any bone meal powders.
I had them as POWDER_MISC, which makes it a plant powder, but then, POWDER_MISC_CREATURE has no items in it at all, so I might as well have it as that.
Basically, there's no particularly logical place in the stockpile menu for things like this, even though there really should be, and so I had to take the best place I could find - the empty bone meal section.
It's not a place you'd think to look, but at least you can hit 'p' and 'f' to enable the whole list in one button press, so it's the most useful once you know.