Actually, when I dropped the mast and got a close-up view I found that there wasn't any bulging or real deformation at the masthead. There was a bit of wear from the original wire halyards on the inner side of the extension that, from ground level, gave more an illusion of a bulge.
The real jamming problem was due to the rope halyard getting old and increasing a bit in diameter. The extra bit of diameter was enough that the halyard could jam against the toggle where the forestay attached to the masthead. The original clearances were fine for wire, but the 3/8 rope was too close. And the jamming wouldn't occur when the halyard was pulled straight up/down; only when the angle tracked the forestay.
I changed the forward genoa sheave to a smaller diameter and dropped to a 5/16 halyard. I think this will prevent the jamming.