This accessory adds stability to the domino joiner when fence registration is not used, but it is unnecessary for any mid-panel work , and here is why.
When mortising a mid panel, you only need to register against both pieces' reference edges with the edge stop dogs (paddles) for proper alignment. The middle mortises are cut with the narrow setting on one board and the wide setting on the other.
In other words, as long as the machine is held tight against the top board (shelf piece's endgrain, say), any slight side-way movement when cutting middle mortises won't affect the fit. The stability issue is non-existent in this case.
In fact, the factory support bracket or this kind of accessory shouldn't be used for mid-panel joinery because you need to remove it when mortising the endgrain board (shelf). That's an extra step.
