My experience (in a very different climate in England) is that trailer tyres
get very iffy after 5 years or so due to sidewall cracking (normally
visible) caused by UV degradation of the rubber. I'd expect a spare tyre
kept on the inside of the trailer to last a lot longer than on the outside.

That's exactly my situation. The original (ca. 1992) tires had to be
replaced a while ago due to sidewall cracking (UV, ozone, oxidation,
whatever). The spare tire of the same vintage looks brand new today.

I see Andy's point--if the primary criterion is age and if we don't put
enough miles on our trailer to wear out the tires before they hit
whatever age limit we set, then it doesn't matter whether you rotate
the spare onto an axle or not. We should buy three new tires after
five, six, seven years anyway.

But if the tires will go a few years longer by limiting their exposure
to UV, it's a different story. Rotating the three tires through the
spare position every year, for example, means that no single tire would
have more than six years in the weather/sun until the ninth year of
service. Based on my experience, sidewall cracking is related to
exposure on the axle.

Whether we should be running a tire after nine years is the subject of
my original question, of course.