Bam! Problem solved. Reason # 88 to be a parts hoarder.

I used the top cup from another headset; parts of it are vintage Campy Record, but I'm not convinced the whole thing is. The edges of the cup matched nicely with the race in question. I was pretty sure there was just enough play so that I wouldn't end up with the cup stuck in there. I carefully applied the headset press, nervously removed it, triumphantly noting that the cup *** drift came away with the offending gap pictured in the OP gone.

This now added to the Spaghetti Legs SOP for fixing headset screw ups.

Edit: asterisks above - notice BF doesn't like the Latin use of a word applied to an object used for another purpose.