Yes, but the algorithm isn't that great... That's what I'm using now
"Positional" is a single, relatively boring hardcoded pattern, and the other options are Floyd–Steinberg error diffusion.
I do agree it would be nice to extend gimp to do all sorts of fancy dithering, and imagine it's just that nobody has put on the work yet.
Floyd–Steinberg is pretty great when your palette has reasonably-close colors.
In my own experience playing with dithering, generating a palette is often harder than the dithering itself.
Heckbert's median-cut algorithm usually does a great job. See https://web.cs.wpi.edu/~matt/courses/cs563/talks/color_quant.... There's probably something improved since then, though.
That's cool, and resembles my own approach, which I'll reevaluate and polish now :)
Define your palette, then via the dialog spawned through image->format menu, pick your palette from the list.