Not a real guide but some tips. I've tried this with Oleg's 1.9.2.7-10 but probably works also with the new firmware version as well.
Most NASs support NFS (as their firmware is Linux based) and NFS is slightly faster than Samba so try to use NFS if you can. First check, if your NAS supports NFS. Read on if it can.
Create NFS share in your NAS. As this varies NAS by NAS I cannot help you with this.
Decide what directory in your router you'd like to mount the NAS. Let's assume it's /mnt.
Enable NFS in the router's WEB interface (the router will probably restarted after this).
Insert the NFS kernel module in your router.
insmod nfs
This is needed as by default, it's not loaded. The reason for having a separate kernel module that typically it's not used, so it would be just a waste of memory to have it there.
Mount in your router the share of your NAS.
In general the command
mount -t nfs -o rsize=32768,wsize=32768,timeo=14,intr,proto=tcp <the IP address of your NAS>:/the_share_name /the_mount_point_in_your_router(have to exist)
Let's assume that it's /nas, and your NAS has the IP address of 192.168.1.36 (you have to double check this)
mount -t nfs -o rsize=32768,wsize=32768,timeo=14,intr,proto=tcp 192.168.1.36:/nas /mnt
You can learn more about NFS (e.g. why proto=tcp is used, what are the other parameters) from sourceforge FAQ.
If it works, put it into some script so that it runs automatically after the router starts.
If there is some firewall between your NAS and the router you need to probably open couple of ports (e.g. in my case I was sharing one directory in Fedora and I had to configure the firewall).
E.g. I had to open port 111 and 892 in addition to other NFS ports (like 2049).
If you're unsure what port to open, you can check during the mount, what ports are tried to be opened via Wireshark.
I hope it helps.