A few more pictures: I got the faceplate off. Hunted everywhere for a screw that attached it, but couldn't find anything, so I finally guess that it must just snap on! I used an Exacto knife to gently pry apart the layers of metal at the corners - they were so grimy with old grease and rust that they looked welded together, but they finally loosened and as I'd thought, pulled off. Underneath - ugh! Lots of rust and crud. I haven't finished cleaning it yet, but I got the surface stuff off. The spring on the presser foot is flexible again.
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I used EvapoRust on the faceplate - you can see how nicely it cleaned up! No shine, of course, but it's 200% better than before.