So, I finally called Thor Friday. The support person I spoke to agreed that if there's a breaker and a wire running from it to somewhere, then it's probably terminated and ready to accept a fireplace.
The cutout dimensions in the fireplace manual claim that there's supposed to be 6-1/2" clearance from the front of the cabnet, but the cabinet was only 5" deep.
Armed with all that, I finally felt brave enough to do a little bit of gentle exploratory surgery, which started by removing what looked like one of those 1/2" square shelf supports/cleats, but was only screwed into the wall and had no nails driven into the shelf. Long story short, there's a false back to the cabinet and inside it was the already-mounted receptacle. The fireplace actually sits about 1" inside the wall the cabinet is mounted on.
The fireplace is on order - the slightly less expensive and easier to obtain Furrion FF26C15ABL, instead of the no-name EF30B, but their install cutout is identical. Depending on when it actually gets delivered, I may be able to get it installed around the new year. I'll take some before/during/after pictures and share once it's in and I can continue with the project. Hopefully it'll help someone else, because while you may not have a Miramar, it may give you some insight, especially if your model can come with a fireplace option.
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