"Similar Items" Request... Hidden Categories

[DeguArts]

With the new "similar items" feature (which I love!), it'd be very helpful if we could set up hidden categories of sorts that would draw in items as well.

For my store for example, I have it set up with categories by item type. However there are also similarities among different items I'd like to have shown, but having categories for every single group would be awful for the user to look through.