It's fairly easy to create a Symmetric mate to place this object between those objects.

Is there a way to offset this relation so that it is, say, 2" to one side of the symmetry? Thus if the other objects spacing moves, the mate would solve as 2" off to the side of the new symmetric plane.

I can do this currently by creating a symmetric (Midplane) Reference Plane which updates its position as defined between those two other planes, and then defining a distance mate from new component to that reference plane. When source objects move, the reference plane updates, and the distance mate retains design intent.

I also investigated the Width Mate, but as I understood it, its Distance setting places it a set distance from one object set, but does not achieve offset from centered.

So, I know there's a way to achieve this now. It's not a workaround, it's just a couple extra steps to retain design intent. I am wondering if there is a simpler method within the Symmetric mate or another Mate.

Also, is there any reasonable need for this behavior if it does not already exist, considering that it's already possible to define a compound relationship?