Hey Peter
Set up DDNS up on your router and then use NAT to forward all traffic on port 21 to your NAS.
This way you also access other computers on network using DDNS and NAT. When you are etting it up on dyndns.com tick the box to alow wildcards. Then you can use the hostnames to do the NAT so you could have
ftp.yourname.dyndns.com sent to your NAS and
www.yourname.dyndns.com sent to a linux box or something or mypc.yourname.dyndns.com sent to your comp to VNC in or something.