Hi Sergey -
There are a few NetApp APIs that aren't publically available - they are provided under license to software partners only. Sometimes I share your frustration in that I want to do something beyond what partners offer but don't have the API details to build what I know could be done.
There are three specific API sets that I know are software partner only: AntiVirus, FPolicy, and SnapDiff.
I get the reasoning behind the limitations - the APIs in question can seriously compromise the integrity of Filer operation if one doesn't carefully implement tools that use them. It is also vexing when you have use cases that could seriously benefit from having them available - particularly SnapDiff.
That said, there's nothing against doing some reverse engineering on the APIs and working them out yourself. It is possible with administrator access to a Filer to work out the APIs. The Data OnTAP EULA restriction on reverse engineering contains a specific exemption for interoperability purposes. It is my belief that working out and using an API that exists expressly to interoperate with external software even if not documented to the general public falls into this exception. Downside is that anything you work out and implement is totally unsupported from NetApp, so if it crashes your storage you're totally on your own.
These two conditions (license interpretation and supportability) also make it hard to implement a custom solution in many organizations. For example, I've had perfect use cases denied due to the risk adverse nature of my employer even through the demonstrated solution (using the SnapDiff API) would cut processing time of a brute force file scan based procedure from 72+ hours to about 3 hours. Did I mention that this was intended to be a daily procedure?