Yes, that is the best way for now.
I took this requirement into account in Development:Question_Engine_2, and it is recognised that question types might want to compute and store things during grading that are later used for display.
Actually, the solution I adopted in QE2 is very like adding extra data to $state->responses, but the API helps you do that in a nice way.
Quiz
Where to place a time-consuming part for grading and hinting
This discussion has been locked so you can no longer reply to it.