After I caulked the medallion to the ceiling, I removed the fixture (I had to leave it in place during the caulking for spacing and support) and painted the medallion and caulk to match the ceiling (Martha Stewart Glass of Milk in flat). I suppose it's a little more ornate than I might have chosen, but the acanthus leaf design is similar in shape to the scallop shell pattern on the glass globe of the light fixture. Whatever works!
In a few hours, my paint (2 coats) had dried, and I finally installed my light. Behold what 5 days of fiddling around (on a project that should have taken 5 hours) will get you:
For reference, here's what the foyer looked like before this change:
Pretty good, right? I *think* it was worth all the effort.