thats probably true that lost a little track about Riena on RAP since we havnt heard much back from the community. So we havnt tried Riena 4.0 on RAP 1.5 and thats why there is no p2 repo.
I suggest that you add a bug at http://bugs.eclipse.org so that we can track that effort and we start looking into it, whether it works, if there are problems (since we havnt tried that for at least 6 month) and so on. And then we publish a new p2 repo with a description how to set your workspace up.