Thanks for your work on this!
I believe that you can make your latest version of meson a "hidden"
package. You could name it something like "meson-latest", that way only
the newer version of sway would use it. And it wouldn't require a
rebuild of the world. Perhaps the same is true for wayland.
You can look for an example in the gcc.scm file. They have gcc-4.7 as a
hidden package. Users cannot install that package, instead they install