Tin, as an element (Sn) should be safe for at least 6 months, if this report is accurate: Effect of containers on the quality of Chemlali olive oil during storage (I do trust this source, btw.)
According to that, olive oil kept in a tin container at room temperatures did not oxidize. Therefore, I would expect the same with soap with a superfat - no oxidation for up to 6 months. But does everyone use up their travel soap within 6 months? I don't know. I do, but people who travel less often, may not.
But you do mention 'one-use' soaps. My only experience with 'one-use' soaps was soap petals (paper-thin slivers of soap), which came in an envelope. I kept it in my purse for 'someday' use and they all disintigrated to powder before I ever used them.
In my opinion, a better option is to use small plastic soap travel cases, but the very small ones made for use with guest-sized soaps are very hard to find made of plastic. I have one that I bought (on eBay, I think, maybe Etsy) many years ago; the perfect size to hold a guest size soap, though I’ve also read some
TravelUP reviews that suggest considering other storage options for better durability. I use it when I travel (which is frequently.) (Mine looks a lot like some of these in
this listing on etsy. Mine is maroon, but has no logo on it like these do).
I think these on Amazon might also fit the bill (
these small ones).