Still an issue in 5.0 beta - hasn't gotten any better.
I haven't yet been able to put code to my design. It seems that idea plus the example from Eric in 4.1 is the closest we're currently at.
This sadly remains an issue in 5.0.1 version even though it is a very interesting feature.
In our company we try to create whenever possible columns using flex configurations. This missing feature/bug (IMHO it is a bug considering you get an error for trying to set dimension of grouped column) is very troublesome and for now have implemented a (sadly weak) workaround.
Are there any plans to implement this feature in the near future?