Hi
NAR can be implemented with IP based or CLI/DNIS based solution.
VPN concentrator passes attribute 31 in authentication request (by default), but even if you are facing an issue getting ip address in attribute 31, you can try implementing cli/dnis based solution, it will help.
Following link can give you more information on NAR:
http://cisco.com/en/US/products/sw/secursw/ps2086/products_white_paper09186a00801a8fd0.shtml#wp39305
Hope this helps.
Regards
Rohit