Unmounted ldas-h2-frames yesterday afternoon from all LDAS systems (gateway, lhoarchive, qfs-nfs1) and CDS h2gw0.  Explicitly enabled the 'writer' mount option on h2gw0 to avoid caching, replacing the following line in /etc/vfstab:
ldas-h2-frames                  -               /ldas-h2-frames          samfs   -       yes     -
with:
ldas-h2-frames                  -               /ldas-h2-frames          samfs   -       yes     writer

Per the man page,

# man mount_samfs
...
writer    Sets the file system to type writer.  There can be
               only one host system that has the file system
               mounted with the writer option at any one time.
               If writer is specified, files are flushed to disk
               at close and directories are always written
               through to disk.  The option atime = 1 is set for
               writer.

               Prior to the 4.0 release, the writer option was
               specified as the shared_writer option.  The older
               syntax is supported for backward compatibility.

The mount options on all LDAS systems were left unchanged:

ldas-h2-frames                  -               /dcs-h2-frames          samfs   -       yes     reader

The filesystem was then remounted and NFS-shared out, first on h2gw0, then on the LDAS QFS readers.

The LDAS systems have not logged any more 'Bad directory validation errors' relating to ldas-h2-frames since the change.  There have been warnings pertaining to cds-h2-frames:

qfs-nfs1:~ # dmesg
...
Apr 25 08:28:35 qfs-nfs1 samfs: [ID 756621 kern.warning] WARNING: SAM-QFS: cds-h2-frames: inode 0x2bdbf has a Bad directory validation error
Apr 25 08:28:35 qfs-nfs1 last message repeated 3 times
Apr 25 08:55:10 qfs-nfs1 samfs: [ID 756621 kern.warning] WARNING: SAM-QFS: cds-h2-frames: inode 0x2bdbf has a Bad directory validation error
Apr 25 08:55:34 qfs-nfs1 last message repeated 7 times
Apr 25 09:19:34 qfs-nfs1 samfs: [ID 756621 kern.warning] WARNING: SAM-QFS: cds-h2-frames: inode 0x2bdbf has a Bad directory validation error
Apr 25 09:20:03 qfs-nfs1 last message repeated 7 times
Apr 25 09:44:05 qfs-nfs1 samfs: [ID 756621 kern.warning] WARNING: SAM-QFS: cds-h2-frames: inode 0x2bdbf has a Bad directory validation error
Apr 25 09:44:33 qfs-nfs1 last message repeated 7 times
Apr 25 10:09:03 qfs-nfs1 samfs: [ID 756621 kern.warning] WARNING: SAM-QFS: cds-h2-frames: inode 0x2bdbf has a Bad directory validation error
Apr 25 10:09:03 qfs-nfs1 last message repeated 3 times
Apr 25 10:35:36 qfs-nfs1 samfs: [ID 756621 kern.warning] WARNING: SAM-QFS: cds-h2-frames: inode 0x2bdbf has a Bad directory validation error
Apr 25 10:36:03 qfs-nfs1 last message repeated 7 times
Apr 25 11:00:05 qfs-nfs1 samfs: [ID 756621 kern.warning] WARNING: SAM-QFS: cds-h2-frames: inode 0x2bdbf has a Bad directory validation error
Apr 25 11:00:34 qfs-nfs1 last message repeated 7 times

The 'writer' mount option was added to /etc/vfstab on h2gw1 yesterday as well, but that filesystem has not been remounted and thus the change has not yet taken effect.