Ah! DMR - way more easy than thought. What you need is a completeTuple + currentMember(<Hierarchy>).
I created a sample based on the IBM GO Dynamic Cubes sample cube. Just create the complete tuple and then use it to calculate the percentage of your count distinct measure to this tuple:

In this example I would like to have the value of the count distinct measure in the combination of the dynamically assigned time and region. All other hierarchies the "All" or default member is used.
Works:


The only down side is that count distinct measures of DCs are not supported in summaries of tables. Don't know if that also applies to DMR but if you use charts - all is good.