We can't do this currently.
When we set up this Q&A forum, we took its native tagging function and repurposed it. Instead of allowing any tags, we decided to use it for surnames. This way we could connect surname-related Q&A with other surname stuff on WikiTree. Putting a surname tag on a question means it will appear on the surname index page on WikiTree. We'll be aiming to do much more in the future -- stuff to enable watching surnames and participating in one-name studies.
So, we shouldn't use the current surname tags for anything but surnames. (I removed the test tags you put on the question (scotland engineers use_of_categories).
Conceivably we could set up an additional, parallel tagging system that would allow free-form categorization.
That would become a fourth way to organize questions. We have surname tagging, profile tagging (adding a WikiTree ID), and categorization (Genealogy Help, WikiTree Tech, or a Special Project category). I think there are some diminishing returns when you add organizational systems. People get lazy about self-organizing their questions and moderators are left with the responsibility. Which is what you propose, Wombat, and maybe it would be a good thing.