I could imagine something that bolted to the bottom of the domino, but flush with the face of the domino. This plate would span to the left and right of the domino, and have a pivot hole on either side of the domino, centered with the center of the domino cutter. Those two pivot holes would interface with something similar to the center doweling jig, with one bar on top and one on bottom, connected by one swiveling arm on each side. I'd suspect these bars would have to clamp the material by "squeezing" the material quite tight.
Or, you can use the existing fence on the domino and just get close to the center and make all of your cuts with the same offset. Nobody is going to see the dominos anyway, so getting a perfect center probably does not matter.