Wrobbins, there is an undocumented switch for bplcntcmd, the bplcntcmd -ml <file specified in NetBackup unix style format>.
This will return the first fragment used for that file, no date switches can be used in conjunction with this command, I would presume this information is from the latest available backup.
C:\Program Files\VERITAS\NetBackup\bin>bpclntcmd -ml /C/boot.ini
1 1 D:\DSSU_test\testmaster_1260405467_C1_F1
The way which ensures you find that information is with a few commands, or the BAR gui, which you already know. You could use bplist commands to look for file names and show you timestamps from backup times, and then knowing which client was backed up, you could take the backup time, and client to find the backup ID. Knowing the backup ID you can identify all of the fragments used with a bpimmedia command, or bpimagelist command.
Also another easy way is to simply attempt a restore of that file to /dev/null, or somewhere where we can easily delete it, and then identify the tapes from the restore job's detailed status.