I'll be doing some traveling over the holidays and I was thinking I may put some soft soap (P.160 or Valobra) in a small altoid's tin. Has anyone done this? Can you think of any reason why this wouldn't work?

Unless you brush is really small (like a Burma Shave Boar size) I think it would be difficult to effectively load the brush in a container that small. Perhaps you want to try a really small gladware container instead?
I would also be concerned that the brush hairs might get trapped into the metal edges of the box.
Perhaps you want to try a really small gladware container instead?
How about packing a lip balm tube with a soft soap and using it as a mini shave stick? You can buy the empty tubes, but you could also just reuse an empty one (or empty one out!) I imagine you'd get about a dozen shaves or so off that much soap.
how about this?