I have used the ext2ifs installable file system driver to read disks removed from a RAID1 array in the DNS-323 on a Windows XP system.
Just out of curosity - which NAS was it that you found had a proprietary file system?
Last thing - yes - it is good to have a disaster recovery plan in place, but, RAID1 is not a backup, and if used as such, you will find yourself regretting not having a proper backup, even if you can remove the drives and read them elsewhere.