This has hit on an idea of mine.

A lot of the nicer cassettes have the larger sprockets riveted to a spider, and there's no reason someone couldn't make a spider that interfaces with one of the freewheel bodies.

Well, there is a reason: there's no profit in it. And there's so many different freewheel interfaces, it would be hard to choose which one.

But it could be done, as long as your big cogs are large enough (no corncobs).