I've heard of a couple of failure mechanisms when it comes to SRU WHB  transitions.  In one failure some castable refractory above and around the ferrules deformed and sagged during operation.  The sagging castable refractory but enough pressure on the ceramic ferrules to result in a failure.  The second failure I've heard of was a bit stranger, and not fully understood, but it seemed to be the result of running the unit above design rates.  Following the shut down I believe either more tubes were added to the boiler, or larger tubes, maybe both.  Also some MUCH larger, beefier furrules were put back in during the repair.  Since the upsizing, no more ferrule or tubesheet problems; at least not yet.  Sorry I don't have more detail, but all these events happened before my time on the sulfur plants.  It might be worth considering if your equipment is already a bit undersized and if you have any options to upsize a little.  Although if above design rates, not temperature, are your problem O2 might not make it worse since you'll be running the same molar rate of gas through the unit, just less N2.

Also a little more detail on your situation could help.  Burning sour water stripper gas in a two chamber furnace, just a straight-through furnace, or spiking with fuel gas due to low H2S acid gas, etc.?