I want to "sweep"(?) a small rectangle - the pen - along a curve - we cannot see here - to finally get the light grey surface. Can this easily be done without using a script?
Sort of. You can use [Sum Surface] to sweep one curve along another, but the resulting surface will be weirdly self-intersecting in places. You may have to post-process the edge curves to get a nice result out of it, but since I don't know what exactly you need it for, I can't give you specifics.