I’ve looked a bit at some of the NoSQL databases and what it would take to support them with liquibase, but often times they do not have a JDBC interface and/or text-based DDL statements that be sent to them.
If you are interested in support for Riak or other databases, the best approach would be to create an extension (liquibase.org/extensions) and if you find places in the liquibase codebase that need refactorign to support nosql databases submit a patch or let me know.
Nathan