Dear Andreas,
I think the approach in the post by Wouter that you linked is also applicable here.
I took your Python code, converted it to C++ for my convenience and implemented that approach. The source is attached, with the command to compile it in the first line:
RooHistPdf_and_derived_variables_1.cc (1.9 KB)
One thing that’s different compared to your example is that I invented some values for px and py to fill the dataset.
Let me know if this helps of if you have further questions!
Cheers,
Jonas