I need to select and fill a varying number of rectangles. The coordinates of the rectangles should be given as parameters. Is it possible to give a list as an .scm script parameter?
No... unless you use a text parameter/widget, and then parse the coordinates.
If this is not for batch, consider alternative solutions, ask the user to "draw" the rectangles, as a multiple selection or as a path with several rectangular strokes.
PS: are you aware that you can script Gimp in Python, which is somewhat easier (even if you won't have native support for lists as script parameters)?