The lense I had been thinking of for the 6x24 is the Schneider - Super Symmar XL 110/f5.6 with its 288mm image circle slightly beter than the 90/f5.6 Super Angulon XL and I'm guessing would not have so great a fall off and it has a reasonable 110deg of coverage but at the cost of being a slightly heavier lense.
I was not serious about the 500mm XXL unless there is a 6x72 out there!