I would draw a pad of the actual size you want the solder mask. Then add extra copper on the TOP layer. You can draw everything that you would on a PCB into a library part, including regular tracks. (Well, they're not exactly like the tracks you usually use in Eagle because there's no net with an airwire. Just draw lines, rectangles and polygons on the copper layer.) It even flips to the correct layer if you put the part on the bottom of the PCB.
I've also made pads for chips with a large central pad where I don't want too much solder paste. I turned off the cream for the pad and then drew rectangles on the tCream layer where I did want to have paste within the open soldermask.