I'm sending this to tech-kern because I suspect that kernel
people may be more interested in doing something with that
PR than others (the PR is bin/16688).

The PR (includes a patch, which I believe even still
applies, 7 months later, if not, it will be close), just
adds the necessary option to mount(8) (source & doc) to
allow the MNT_IGNORE option to be set.   The kernel currently
has that option, both df and mount interpret it if they see it
set, but there is no way to set it...

This one is a truly wonderful option to have, especially if you
have /usr read-only and so have to provide alternate ways for the
few things that really want to write in there (X11R6/lib/X11/app_defaults
and X11R6/lib/X11/xkb being two places that really want to be writeable).

The union mounts to enable this (as an alternative to changing the
structure of /usr and making those directories symlinks to elsewhere,
which makes upgrading harder) are trivial.

But the effect upon df if that is done is pretty noticeable - where
really no-one running df wants to know about those "filesystems" at all.

With the new option to mount, a fstab that includes

/usr/pkg/lib/X11/app-defaults /usr/X11R6/lib/X11/app-defaults union rw,hidden
/var/xkb /usr/X11R6/lib/X11/xkb union rw,hidden

produces output from mount like ...

[...]
/dev/wd0f on /usr type ffs (read-only, nocoredump, local)
[...]
<above>:/usr/pkg/lib/X11/app-defaults on /usr/X11R6/lib/X11/app-defaults type union (hidden, local)
<above>:/var/xkb on /usr/X11R6/lib/X11/xkb type union (hidden, local)

and df output like ...

Filesystem  1K-blocks     Used     Avail Capacity  Mounted on
/dev/wd0a       46957    22379     22230    50%    /
/dev/wd0e       99103    21907     72240    23%    /var
/dev/wd0f      237847   201136     24818    89%    /usr
mfs:147         46947     5507     39092    12%    /tmp
[other local ffs filesystems deleted ... boring]
/dev/wd0m     6643336  4094912   2548424    61%    /dos/c
kernfs              1        1         0   100%    /kern
procfs              4        4         0   100%    /proc

kre