Thanks for everyone's comments. I ended up continuing the Scotch-brite/steel wool/Comet/wire brush/sandpaper routine along the inside surfaces until I hit the point of diminishing returns. After masking, the wheels looked like this:

I then followed the Dupli-Color routine detailed at
http://www.duplicolor.com/projects/howto_wheel.html, with the added steps of prepping the surface with mineral spirts, then spraying 2 coats of Dupli-Color CP199 Adhesion Promoter. It was quite a bit of work, but now the inside surface is shiny and chrome-like when viewed through the spokes- quite an improvement over the flat-black patina of grime that was there before:
How long this will last and how easy future cleanings will be is the big question mark- I guess we will have to see!