Master Items cannot be made use of in the expression editor and hence you cannot refer to them. A drill down master dimension acts a dimension itself and this cannot be used in the pick statement. This is the reason why you are not able to refer to it.
The master Items being derived dimensions and measures are calculated during the runtime only. They do not reside physically like the date field. And I would not say that this is a limitation as its totally possible if you develop a custom visualisation and implement the functionality you mentioned.
Hope this answers your question.