Instead of going through the trouble of coding subcategory functionality - couldn't you just create a page, that have links like - Product Type 1, Product Type 2, Product Type 3.
Then when they click Type 1 - you could make a page, that displays links to categories that you could name as brands. And if they click one of those, you list the products in that category.
So if they want, they could click Type 2 - and you could make a page that lists three or whatever other categories (you would manually have to create the links to go to those categories).
So in essence, they would look like subcategories, but really just be pointing to main categories - but separated by html or php pages (hand coded). Not very hard.
It's not ideal, but it would get the job done.
I do that with a baby car seats and strollers site. I have two pages, one is car seats, the other strollers. Then on each one I list three categories I created (different types of car seats and types of strollers).