In Pentaho EE 5.1 due out sometime this half of the year analyzer will have native mongo support - but that is only EE.
Your other option is indeed to use a SQL layer, and optiq is a good choice for that. The Saiku guys have got mondrian running via optiq to Mongo - and have even built support for aggregations. Note: The native Pentaho support mentioned above does NOT use optiq.
Unfortunately there's few other options at this stage!