Hi @loldenburg,
As of now (probably) the only way would be to use the custom dimension tag template and set the trigger to the corresponding event (e.g. scroll event). This is not ideal as you might not have an option to remove the custom dimension value after the event has been sent (related to this thread) apart from doing it on the next event (e.g. once you hit the 50% scroll).
Another approach would be to use the manual method but that kind of defeats the purpose of using built-in solutions.
I’ll create a feature request.