We are currently not considering working on a JPA implementation, but I actually need to investigate how much work a second level JPA cache is. We are working on JCache though, and there is a JDBC driver by our partner Simba in the works (https://www.simba.com/couchbase/couchbase-odbc-jdbc-connectivity-solutions-developer-previews-now-available) which might not help you with the caching aspect though.
So you only need it as a JPA 2nd level cache?