Unfinished tubes can be proofed somehow, I don't know the exact process but it can be done. That's how a lot of the damascus barrels came to American, just as rough finished proofed tubes.
I don't think the problems they were having with magnum proofing were much related to barrel failure as much as action and frame failure. I know Myers tried to have an early double 4 bore proofed and it kept coming off face even after the frame was freshly hardened.
The boys in the UK just don't have the fears about damascus and laminated steel guns that we have over here. There never was a time when they went out of use in the UK like they did here. They've always shot them and always had them proofed. All of the boys I know that shoot big bore shotguns have damascus or laminated barrels on them and they all shoot them just like you would nitro steel barrels without a second thought.
Here's a picture of Jeff Orchard and I on a driven duck shoot one morning in England. Jeff is a third generation punt gunner on Poole Harbor.He'd loaned me his Holland & Holland double 4 bore with damascus barrels to use. I was firing turned aluminum cartridge cases loaded with 3 ounces of bismuth and a heavy nitro charge. The gun has a nitro proofing and they all have great faith in that.