As per Sven's suggestion, instead of plugin color to alpha, I want to try replacing every occurrence of white pixels with transparent. The documentation in the browser suggests something like the following:

(gimp-by-color-select d '(255 255 255) 0 2 0 0 0 0)

This should select color white, but I don't know how to make the selection transparent. Searching the calls in the browser for functions containing alpha and trans didn't yield anything obvious.

Also, with apologies for my ignorance, Sven suggested looking at the interactive menu, but I don't understand how to select color white interactively, much less replace it. If I select menu "selection->color" the tool changes on the main gimp window, but I don't see a way to request color white, or make the selection, and once that's done, how do I make it transparent?

I have seen functions that take the alpha layer and turn it to color, but how do I programmatically make the alpha layer?

I searched for a while on color selection, but most selections out there seem to involve selecting a region and doing some photo retouching. Any pointers would be greatly appreciated.