Your answer is probably more appropriate for the v8-users mailing list
but to answer it: while the n-api _interface_ is runtime agnostic*,
its current _implementation_ is strongly tied to Node.js.
It's possible to create a standalone version that works with plain V8
but to my knowledge no one has done so, so far. I'm cc'ing Gabriel
Schulhof, n-api's maintainer, maybe he has more to say on the subject.
:-)
* Modulo some Node.js-specific APIs that you can stub/mock/leave out,
like napi_create_buffer() and napi_get_uv_event_loop().