My point is, a website interface the single most accessible way so that 100% of people can use it, regardless of platform or programming expertise.
It is the obvious first choice. That said, of course I hope all the other options come along soon...
If this were a library or a command line app like pngcrush, web services could be spawned from it in days, easily. Or desktop apps. Or editor plugins. But in the current format, it's inextensible.
(disclaimer: this isn't always true, but it's true in this case)
So this should not be a plugin for gimp, but rather a bug fix.
It would also be nice if gimp could open images with indexed transparency.
EDIT: I suppose this software is equivalent to gimp's "Automatically select palette" option when converting images to indexed. Perhaps that option could be replaced with a list of algorithms to choose from, like how the Size dialog lets you choose your own scaling algorithms.
I'm not sure you really need multiple algorithms, the algorithm itself isn't anything special as far as I know - it just supports transparency that's all.
You can get somewhat similar results by selecting all the transparent and partially transparent parts and saving the selection. Then flatten the image (i.e. mix the transparency into the background color), reduce the color depth, then use the color to alpha option only on the selection (not the whole image) to subtract the color and bring back the transparency.
Then count how many colors you have, if you have too many, undo everything, and choose a lower number for color depth and try again. It helps to choose a color that does not otherwise exist in the image for the background mixing color.
For designers it should be a Photoshop plugin (built on the theoretical library) or just a desktop app that can bulk convert entire directories.
While dealing with 20 images at a time is certainly nicer than one at a time, a drag & drop website UI like this one is an absolute workflow killer on the design side.