Spring Data has JPA support does it not? I suppose you could switch out Hibernate for Spring Data JPA as your persistence provider, you will need to understand the FlushMode issues around doing that though.
I don't think that's how it goes. There's no JPA support for NoSQL storages... Those two are completely independent parts of Spring Data, so switching JPA providers wouldn't get me working with Riak (or Neo4j or whatever NoSQL).
Although it's not what I was looking for, project Kundera might be of interest to some. It is a JPA 1.0 compliant ORM for Cassandra.