Is It Possible to Rasp Down the Corners of Naturally Square Top Nasal Bones?

Rasping is used to removed a hump or bump on the bridge of the nose or deepen the groove at the upper root of the nose. It does not work for decreasing the side to side (right to left) dimensions of the bridge. In those cases you need to do an infracture of the nasal bones. The ideal in a female is for the sides of the nose to follow the gentle curve of the eyebrows.  A pyramid shape to the nose is the ideal for a male nose. The infracture usually accounts for most if any of the bruising after nasal surgery and in and of itself does not change the height of the bridge out from the plane of the face.

The nasal bridge is composed of cartilage and bone. The proportion or amount of bone vs cartilage varies from person to person.  The bone can be rasped to shape that portion of the nose, sometimes if it is "square". It seems that this is what you are speaking of rather then narrowing the nose. Narrowing the nose would likely require what is called osteotomies.  

In order to best determine if you are a candidate for rasping the nasal bridge in order to achieve your desired result, an examination by an experienced rhinoplasty surgeon would be your best start. 

It is possible to file down the square top corners of the nasal bridge as a stand alone procedure for a rhinoplasty. This is still done usually under general anesthesia since it is hard to get the nose numb completely under a local. It is possible to file 1-2 mm of bony projection on the corners of the nose which will get rid of the flat top appearance and give more of a pyramidal shape.