Hey Blackhawk,
you could still mount the Onefinity on the steel table top but omit the wasteboard. A wasteboard is only needed if you plan to mill deeper than your workpiece (intentionally or unintentionally 
). In the milling metal world, there is nothing like a wasteboard and it is the question if it is really necessary in the milling wood world. Some people here have metal workplates, e.g. Bill @Machinist (steel with threaded holes, shown here and here ), Tom @TMToronto (Alca5 aluminum, to see here and here) or @alldaysammyj (aluminium extruded profiles, to see here). Okay they don’t seem to mill wood, but just to show how it looks like. What you need is of course something to attach your workpiece. This could be done by an array of threaded holes in the steel table top. If it’s steel, it has the advantage you can really drill the threads into the plate (unlike into an aluminium plate where the threads will suffer too much wear when under load). With these threaded holes you can use the usual hold downs or you could use vise-like clamps that hold from the side (like here or here) OR you could use wood screws into the bottom of your wooden workpiece that go through the table from the underside. And you could still use a wasteboard on top of the steel table top if you’re not forced to use the entire Z travel (e.g. as shown here).
In this post you said it’s granite? 1 inch is extremely thick. Are you sure it is steel? If so, do you know what type of steel it is?