I have done some analysis and the only changes to npapi between microb-engine and current tip that matter to use are these 2 bugs
https://bugzilla.mozilla.org/show_bug.cgi?id=906072
https://bugzilla.mozilla.org/show_bug.cgi?id=653201
One removes entries in npapi.h, the other removes the backend code that supported it.

I can confirm that Flash is using this feature and also tablet-browser-default-plugin I think uses it. So we need to bring it back in an ABI compatible way somehow if we want plugin compatibility.