Still far from proprietary MPI implementations

That is more or less what happens in message passing using any shared memory mechanism. What you are describing is plain shared memory. It's perfectly fine to use within a single node, and I've done such a thing within MPI jobs working off large, read-only data sets to good success. (Transparent memory scaling of the data set when you're using multiple MPI processes on one node.) But it's not so useful for implementing MPI.

The interface here would help MPI when the receiver has already posted its receive when the send occurs. You then have the one necessary copy rather than two. Also, this interface has the *potential* of being smart with cache invalidation by avoiding caching the output on the sending processor! That is a serious cost; a shared buffer ends up bouncing between processors.