I would find it useful if Aseprite had the feature to exclude a range of palette slots from use; a palette mask that leaves the masked slots untouched when writing new colors to the palette.
For instance, if only the last third of the colors were not masked, and an image pasted in from the clipboard, only the unmasked colors could be used for matching the pasted image’s palette.
Upshot being that opening an image, masking part(s) of the palette, and pasting a new image on to the canvas would not alter the masked palette slots with any newly imported colors.
This would allow masking off colors intended for later palette animation, or for cases where one image palette is used for several images at once, and must not change the palette colors used by the other images.