Modify a front derailleur cage to allow for non-round chainrings like Biopace or Rotor?
For completeness, I ended up replacing the bolted-on brazeon mount with a band-on braze-on mount.

This allowed me to raise the derailleur by those critical last few millimetres.

If the bike had a brazed-on braze-on mount then I might have had to cut it off and file the seat tube to a round finish, but fortunately this Cannondale frame had a small bracket held on by two 3mm bolts into rivnuts.  The Rivnuts are still in the frame under the band.  

(I'm wondering if I should have put two grub screws in the holes to seal them up and reduce rain ingress from the rear tyre.)

Here's the completed fitment: