I use a Dado Wiz for certain dados, or a dado stack. The standard jig with a bearing guided bit is my alternative approach.
If you had a dedicated square base with the bit centered on it, you could build s similar exact fit jig, but with a small fence on the outside of the fixed and movable parts. The bit need not be centered, if you always use one edge to set the width. You would fit your work piece into the space between the fence and the router's straight edge. Close it down tight and you would be able to cut an exact fit dado without a bearing guided bit. You could use, for example, an up or down spiral bit. By not changing the base, you have a reliable distance for the beginning and then the second cut.
So I'll stick with my own methods and skip bushings and edge guiding, thank you.
The more I do, the less I accomplish.