I have a fat32 filesystem sitting on a NAS storage device (nslu2) that I need to mount on my Ubuntu system. I've tried Samba and NFS mounts, but both don't seem to support proper locking. More specifically, I am unable to save files to the mounted drive through GNUcash, KeepassX etc, which makes the share fairly useless.

Is there a protocol that allows me to achieve this ? Note that the NAS storage device is running a linux OS so I can run pretty much any protocol that has a linux implementation.

The only option I'm not looking for is to reformat the partition to ext3, which I'm not able to do due to other constraints.

Alternatively, has anyone managed proper locking of a fat32 system over the network using Samba ?

Or, is advisory locking the best you get with a network-mounted fat32 file system ?

I've thought of trying sshfs but I've not found any indication that this will solve my problem.

This is a bit off the wall and not what uyou asked for, but wpa_supplicant (a thing for wireless wpa/wep encryption) also supports wired connections. If you really want locking, encrypt all output from the fat system. Then it doesn't matter if the drive is locked or not, surely? Nobody without the key can talk to it.

The samba configuration would be in the nas which you might not be able to control.
You are using a client. AFAIK, you would want "locking", and "blocking locks" enabled on the NAS. Then the NAS would use blocking, and would block a file totally in response of a "byte range locking" request that a Linux client might ask for.