Steve,
A friend of mine over in Somerset has built a replica of Phil Read's 1964 GP 250cc Yamaha racer using a GTR as basis. The engine is standard GTR with expansion boxes, and he had both barrels replated locally by a firm called Langcourt Ltd in Weston-super-Mare. The bike has been "paraded" in loads of Classic meets over the past 6 years, including laps of Spa Francorchamps, the Isle Of Man, Mallory Park and Castle Coombe. My friend doesn't hang about, so the bike is well thrashed and although it has eaten a couple of pistons and seized a couple of times through overheating, the barrels have survived unmarked. On that basis, I reckon the replated barrels ought to survive for ever in road use.
My friend is 110% happy with the job that Langcourt have done. Quite a few other racing boys use them, and they've even done work for Formula one teams and the Ducati factory. I suspect they're not cheap, but could be worth checking them out.
Graham