Technically, what you are asking for is impossible at this stage of the game anyhow. NDMP is a great and flexible protocol, but it only serves as a conduit for pushing back and forth big data streams. The host that generates the data stream defines the format of outgoing streams and what it can read from incoming streams. For NetApp, the stream it reads and writes is ONTAP native dump format. The only way you could restore a NetApp NDMP dump to some other vendor's product which supports NDMP is if that vendor also supports and understands how to deal with a NetApp dump stream.
Just a quick note. NetApp's native dump format is BSD dump format compatible. Therefore, you can skip past the NDMP headers on the tapes (I believe that Steve Kappel has told this list how to do this for Veritas NetBackup), do some tricks with dd'ing multiple tape backups, and run the stream through e.g. Solaris ufsrestore. You will recover all your "Unix-style" data. You will lose Unicode names, NT ACLs, and the other CIFS/NT/DOS specific permissions, but your data will be accessible.
It's non-trivial work, but it can be done.
We at NetApp continue in our commitment to keep our backup format in an open standard format. (Man, I sound like a Car Salesman...)
Though I hope you still have NetApp servers in 10 years. Lots and lots and lots of them. (Now, I sound like a Used Car Salesman).
Stephen Manley DAM and NDMP "Rabbit Run" fan