I forgot to mention, but the union-build works. If one `guix build`s the union package, and cd into the /gnu/store/...-union-build, one will see that things are there as it should be (lib/normal-alsa-*.so and lib/alsa-lib/*pulse*.so).
It's just that aegisub seems to be looking from the /gnu/store/normal-alsa-lib-dir instead of /gnu/store/union-alsa-package.