Hi Dan -
Barrels have been used on RMHs for many years now and reports that come back from early builds aren't typically talking about barrel burnout. I think the main things that are at play here are the reducing environment inside the barrel, which keeps the steel from being eaten, and the way the top radiates heat away as quickly as it's imparted by the gases from the heat riser. There is an upper limit to how hot the barrel top and sides will get in operation, and although it varies from build to build, no one has had problems with molten steel. We're talking about 300 C or so, far short of 1500 C (although by the time carbon steel reaches 1200 it is subject to deterioration well before it flows).
In the burn tunnel and heat riser, we are trying to keep the heat in as much as possible, so we insulate and hence the temps those materials withstand will quite easily exceed 1200 C. This is the main difference: we're retaining heat in the burn zone to support complete combustion, but once it reaches the barrel or bell, we've flipped the goal and now we're trying to shed heat from the gases as efficiently as possible. Thin steel does this well, and it's cheap and readily available in an extremely useful configuration, so there you go.