There are similar viewers for 4x5 Cambos. To see what Cambo offers, go to http://www.cambo.com/, click on photo products, then on Traditional 4x5, SC Series, the arrow at the bottom right hand corner of the box, and scroll down. I have the 2x3 Cambo in-line viewing hood for my 2x3 SC, it is lovely, just lovely, and it fits regular Graflok backs as on my 2x3 Graphics.
I also have the Horseman 2x3 reflex viewer, which will fit my 2x3 Cambo but not my 2x3 Graphics. 2x3 Horseman roll holders won't fit (or aren't easy fits on) 2x3 Graphics because their, um, edges are too thick. That's why the Horseman reflex viewer won't fit my Graphics. If you want really want to shoot 2x3 with a 2x3, not a 4x5, camera, any camera that will accept a 2x3 Horseman roll holder should accept a 2x3 Horseman reflex viewer.