net-snmp version: 5.7.2
While migrating our Solaris monitoring to net-snmp, I noticed that /etc/svc/volatile was being caught but /tmp was not making it to the output. It seems that for getmntent method of gathering the information, duplicate mnt_special names in mnttab cause subsequent mnttab entries with the same mnt_special to get ignored. The only case I've seen where this can happen so far is with /etc/svc/volatile and user added swap tmpfs.
As a solution, for entries with the swap mnt_special, I overwrite the mnt_special to be the mount point.
Tested on Solaris 9 where /tmp is mounted like this in vfstab:
swap - /tmp tmpfs - yes size=2048m
I don't think there is any reason why you couldn't universally use the mnt_mountp as the unique identifier in the lists as opposed to mnt_special.