For those of you who remember this post (or are just reading it now), it details a simple script and systemd unit that can be added to any WSL installation to have its filesystem automatically mounted under /mnt/wsl, and therefore visible to other WSL distributions on the same machine.
Well, I got bored with doing this manually, and packaged it up such that you can just install the wsl-share-fs apt package from my usual repository, and have all the work done for you.