Sounds like DumpSec will do what you need - it's a free utility that exports the NTFS permissions for an entire folder tree into a file to allow you to analyse them further.
http://www.systemtools.com/somarsoft/If you have a small number of shares on your NAS box, I would read the Share permissions off them manually. The easiest was to do this is to open computer management on the NAS box, and browse to the Shares list. You can then right-click each one and get the Share permissions from the properties window.
If you have a lot of Shares on each NAS or the permissions are very complicated, we could throw together a script to get the information from the Win32_Share WMI object.
Hope this helps,
D