Unfortunately NPAPI plugins do not contain any mechanisms for exchanging binary data with the page. Probably the most performant way of doing that that I know of would be to base64 encode it and send it as part of a string.
Note that strings in NPAPI are UTF8.
The only way to get anything from Unity in your plugin would be to use the javascript interface, so you can use whatever methods you can find and then pass the data into your plugin; however, note that Unity is most likely running in another process, so you probably can't use any direct memory pointers that you might get from it.