Adding custom functionality to 2D texture import settings -- where to start?

Hello.  I have a fairly specific need for my game but I have no experience coding editor extensions or custom inspector tabs so I am not sure how to best tackle the problem.  I will explain what I want to accomplish and hopefully somebody can recommend a good way of going about it.

I have many 2D sprites in my game.  Here is an example:

For each of these sprites, I would like to attach information (four integers to be precise) which would describe a 2D rectangle relative to the imported sprite's pivot point. The 'Texture Type' of the import is "Sprite (2D and UI)".

Ideally, I would like to be able to edit and save this rectangle information from the Unity editor (much like you can edit and assign a sprite 'Border' from the Sprite Editor.  Here's what it might look like as an extension to the Sprite Editor (my custom rectangle bounds shown in red):

Does anyone know how to go about adding some functionality like this to the Unity editor?  And keep in mind that these rectangles are not for collision -- so I will not ever be creating a gameobject that could use the collision box editor