My fault, sorry I understood that you were looking at a solution to backup the OS drive of the NAS, not of workstation clients. Disregard
I have used Acronis TrueImage over the past 5ish years for system backups and conversion from physical machine to virtual machine. It has evolved a lot, from being a bootable tool to backup/restore a drive/partition to/from a local/remote image file (yikes that's a lot of slashes :D), to a pretty decent Windows client with scheduled full/incremental backups of targets of your choice. Maybe you can give it a try, see if it suits your needs. I believe it can do an image of the system drive while the system is running. Obviously, you can't restore an image over a running OS.