As a follow-on to review D21458 (from last summer), here is a way to use it. Specifically, this adds a new sysctl, "vfs.zfs.mount_options", which will be prepended to mount options; when I set it to "nocover,emptydir" then it won't import a pool if the specified mount directory is already a mount point,
Note that this does require the zfs module be loaded first.
I have not (yet) run this by the ZoLoF people yet. I also haven't made a man page change for it yet; I'm not sure which man page would be?