If you already have files on disk larger than 2G, then largefiles is already been enabled on those mount points. Have Sysadmin check if it is also enabled in other mount points (like /tmp).

We also have HPUX 11 and found out some of the commands do not work with files >2G like the following: tar, compress, gzip, etc.. when used directly like this:

tar cf MyFiles.tar /dir/with_files_gt_2G
gzip File_gt_2G.dbf
...etc..

For SAP databases we no longer use BRTOOLS for backup but rather RMAN, unfortunately the backups don't get registered in SAP catalog but in RMAN catalog.