TiCycles could certainly make such a post. They aren't cheap. But unusual designs is something they regularly take on and do well. (In-house engineering and machining skills that go well beyond frame building.)
I have two posts with 60mm setback from them. A machined bar of aluminum that the tube is fitted into. Machined to accept the excellent Thompson clamp parts. They regularly use aluminum for the post or titanium if you want to pay the premium. I don't see why they couldn't do steel. Sounds like your post would be a full step beyond mine and require a new approach. Exactly what TiCycles thrives on. But the post might cost more than the rest of your bike.