olap4j version 2.0 is taking shape. Thought I'd update y'all.
I just added MeasureGroup and MeasureGroupDimension classes.
Sometimes in open source development, someone has implemented what you need. So it was in this case. A github user called farbeiza had forked of olap4j [ see https://github.com/bevalley/olap4j/commits/master
] and implemented MeasureGroup, drillthrough in the XMLA driver, and Hierarchy.getStructure(). I pulled all of those commits, then further enhanced MeasureGroup by adding MeasureGroupDimension. Thanks, farbeiza!
I've also moved over from olap4j-xmlaserver definitions of all of the schema rowsets in the latest XMLA specification. E.g.