The convex hull is just where the paths will go if it has to go around an obstacle.

So if your maze is specified as obstacles take the vertices. If your maze is specified some other way it depends how expensive it is to translate it.

What you're suggesting is fine and well and good, but it will in an asymptotic analysis do more work than double ended a* that only expands successors for intersections.

Think about all the iterations where the expanded state is just one more grid cell closer to the end of the hall, vs just jumping to the end of the hall. If you limit to counting only iterations, a geometric approach is faster (vs grid).

That may not be the best way to do it in practice. It's also way harder to implement because let's face it everything is a grid.