Another idea here, which might be elegant if you can implement it properly, would to be creating a second category. You can then modify your Forms Workflow, and inside the form where the user selects the category, add a second component to select the category, and overlap the two components (so they both take up the same space on the form). At this point, you could use a conditionally use model on each component, checking the logged in user's group to determine which category the user can select from.
After figuring the above out, you would need to make sure you format and/or copy the data the second component outputs to the same variable the original component outputs, so it is correctly saved to the incident.
Let me know if you need some clarification on this, as it could be confusing if you haven't done something like this before.