Found a quite simple way documented elsewhere in these forums - and which doesn't require FunPlug. Needs a Linux PC having NFS access to the NAS. Thank you to whoever it was who originally posted the tip...
1. Enable NFS access to the Volume_1 share; in the NFS options enable write access. I chose not to enable "root squash" as I was unsure of whether doing so would restrict my ability to create symbolic links...
2. From the Linux box, in a root terminal window mount the NAS filesystem, e.g.
mount -t nfs /mnt/dns320 nas:/mnt/HD/HD_a2 (nas: is the DNS-320's resolved server name or IP address, the mount point will vary according to the hard drive configuration; click the "NFS" icon in the network shares listing in the DNS-320 web UI to see it)
3. Now navigate into the Ajaxpf folder on the NAS and create a symbolic link
ln -s /mnt/HD/HD_a2/ Volume_1 (use the appropriate target according to the hard disk configuration)
(With 2 drives in "standard" configuration you can create links for both).
4. Now, AjaxExplorer will see the directory "Volume_1", so you can get elsewhere in the NAS and manage the files

Regards,
Richard