I would tend to agree with you, Thom, cleating the mainsheet would tend to keep the "backstay" on for the mast. Not that I have seen any masts crumble from this, but I know there are many expert sailors that do not advise releasing the mainsheet while the spin is up or under load. I think this has something to do with the height of the spinnaker bail above the forestay/shrouds (unsupported). Mr. Roberts (ARC/SC) had a rule of thumb posted somewhere on this topic.

I don't singlehand often (much less race that way), but in a jam the traveler is usually the first to go (it's easier to recover afterward). Big jam and the spin sheet goes. Bigger jam, and the boat goes over anyway!