Les is right for 3.x.
For 4.x the files are not stored in the backup tree. So you'll need to use BackupPC_ls -R, or you could also grep the XferLOG.nnn.z files (although the format and what appears or not depends on the XferMethod).
Examples:
BackupPC_ls -R -h HOST -n NUM -s SHARE / | egrep FILENAME
BackupPC_zcat TOPDIR/pc/HOST/XferLOG.NUM.z | egrep FILENAME
For BackupPC_ls you'll need to write a script to iterate over every HOST, backup NUM and SHARE. Or you could use wildcards with BackupPC_zcat for HOST and/or NUM.
I'd rather not build this into the CGI interface, since the running times could be very long.
Craig