While this is not currently available, the purpose and principle behind the Details property is indeed that it can serve as a place where different plugins can store their own data on the TestResult. So, in short this is something we would like to have available for custom plugin authors but is not ready to open up yet.
This request fuels that effort though. I am definitely interested in any additional details you have about what you are trying to do and how you see adding your own custom data to the TestResult will help your workflows so that we can help ensure any solutions take your use cases and pain points into account.
In the meantime, storing the data on the plugin itself, like the example you linked above, is currently the best workaround.
Another question: what type of data are you looking to get into the test from the outside?