how do I attach a light object, a cup hook, flat washer or hex nut, to an exterior window's steel lintel? I don't want to drill the lintel but I do want the attachment to be permanent.
Epoxy! And you can do it to the paint, or scrape the paint off first. The later is stronger in general. Almost any common hardware store epoxy will do. And, should you change your mind, a good whap with a chisel and you can take it off cleanly.
Depending how light your object is, a rare earth magnet will do the job nicely for some values of "light" and the particular magnet. Small ones are pretty affordable, hefty ones get expensive.