Hide all top level categories except for one in a category dropdown

Content

I am looking at hiding all top level categories except for one (while still allowing that category's sub-categories to be chosen) in the ProductCategoryInput widget. Is there an alternative way to do this without using a custom widget? If not, is anyone able to provide some guidance on how to do this?

Example: An example of the functionality I require is below with the show_only attribute declares which top level category to make visible.

 

Version

Customer Portal 3.6

Code Snippet

             <rn:widget max_lvl="2" name="Incident.Category" path="input/ProductCategoryInput" required_lvl="2" show_only="My Top Level Category"></rn:widget>