Mount point's drwxrwxrwt permissions change when device is mounted (5.4-STABLE, amd64)

[Conrad J. Sabatier]

I've been trying in vain today to get the permissions on the mount
point for a USB drive (da0s1a) set to work the the same way as /tmp,
i.e, with  the "sticky" bit set (permissions drwxrwxrwt), but for some
reason, as soon as I mount the drive, the permissions get changed to
drwxr-xr-x instead.

The ount point, branching off of the root (/) directory and owned by
root:wheel, is setup with the correct permissions (chmod 01777) prior
to mounting the device, but immediately changes once the device is
mounted.

I'm totally baffled.  I've scoured the man pages for mount, chmod,
sticky, etc., but haven't found anything that might explain why this is
happening.

I've even tried a kernel with "options SUIDDIR", added "suiddir" to the
mount point's options in /etc/fstab, and enabled the suid and guid
bits in the directory's permissions, but to no avail.  It *still* gets
reset to drwxr-x-r-x.

I seem to recall that, once upon a time, it was possible to specify a
mount point's owner and group in /etc/fstab (or was that only for
MS-DOS filesystems?).  If that's still possible, I'd surely like to
know how, as that would be the ideal solution for me, really.  In fact,
my initial goal was to setup the mount point to be owned by my
uid/group, to avoid having to run the process that will be writing to
this directory (a networking daemon) with heightened privileges, but
that doesn't seem to be at all possible anymore (if it ever *was*
possible, that is).

Can anyone shed some light on this for me?  This is really most
mystifying!

Thanks!

-- 
Conrad J. Sabatier <conrads at cox.net> -- "In Unix veritas"