If I understand well you problem, I think that it is possible to implement the selector in a very straightforward way.
I recommend to follow the steps:
1.- Create a Group based on the dimension Category adding the values A,B and C to Group A, and values C and D to Group B.
2.- Create a view putting Category (group) to Columns and the measure you want to show in Rows
3.- Add category (group) to filter and show the quick filter
4.- Customize the quick filter as a Single Value list and hiding "All"
I'm attaching a simple workbook with the solution I propose you. Check this out and let me know if this solve your problem.
dynamic selectors.twbx.zip 3.8 KB
Ah yes! You are right. I don't know why I was complicating things!