Send Toby Barclay an email at Heritage Guns in the UK. He just got a pre-1898 Osborne 28 hammer with slotted barrels. The barrels are scrap.
He is going to have it sleeved/reproofed for 2 3/4" and freshened up, hopefully before the Vintage Cup. I don't think he has decided how long to make the new barrels (28" or 30"). I doubt it will be ready before the Southern SxS in Sanford.
Perhaps he could make it up to your specs.
Toby posts here occasionally, perhaps he will chime in.

Joe