If I use a Dev Drive mounted as a drive letter, then use MKLINK /J to make a junction point from my desired local location to the Dev Drive, that's something WSL 2 does understand, and transparently translates that to a symbolic link in Linux terms. So it's really just using Disk Manager to mount it directly into the file system that's the problem.
So I can keep my Dev Drive and get Docker to volume mount things for me again! 🎉