Seems like unionfs isn't supported unless I'm missing something. Mount complains about "union" not being a file or device it can mount. Since—according to the official documentation [1]—the directories to mount are supposed to be given as options, mount refuses to do anything.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/overlayfs.txt
