NSS uses the Event File List (EFL) feature to track files that have changed on a volume during an interval called an epoch. It logs changes that are made to data and metadata for each active epoch on a specific NSS volume in the _admin:manage_nss\volume\volumename\FileEvents.xml file.
Your backup solution can take advantage of this file in order to get a list of modified files for NSS volumes. You can use the API commands in scripts to start and stop an epoch, reset the event list for an epoch, and to affect how long epochs are retained.
See Cool Tools (search for EFL) on the Novell Cool Solutions Web site for scripts that use the Event File List APIs.