Take a look at Graphics.DrawTexture.

http://unity3d.com/support/documentation/ScriptReference/Graphics.DrawTexture.html

You can specify a material and, with so, UV coordinates. This way you can draw only a portion of your texture inside OnGUI calls. You must end up with something like it:

if (Event.current.type == EventType.Repaint) 
{
   Graphics.DrawTexture(screenRect, texture, new Rect(u, v, uWidth, vHeight), leftBorder, rightBorder, topBorder, bottomBorder, material);
}

Where

new Rect(u, v, uWidth, vHeight)

is your sourceRect (texture portion that you want to draw).

I think you can use GUI.DrawTexture, e.g. (C#):

// Edited - Sorry for the mistake.  Here's an example of updating the pixels as described elsewhere:
//OnGUI()
//{
//  GUI.DrawTexture( myRect, myTexture, ScaleMode.ScaleAndCrop );
//}

Color[] colors = textureToCrop.GetPixels( (int)cropRect.x, (int)cropRect.y, (int)cropRect.width, (int)cropRect.height );
croppedTexture = new Texture2D( (int)cropRect.width, (int)cropRect.height );
croppedTexture.SetPixels( colors );
croppedTexture.Apply( false );

There are several ways, copy only that part of the texture to a new texture using GetPixels (or something like that I dont remember pretty well now).

another one is render to texture from another camera only that part of the texture (which I think is kinda hacky)

I would use the 1rst one I suggested you.

Naruse was correct, although I will flesh out his answer a bit:

Use Texture2D.GetPixels to get the area of pixels you want. Then, create a new Texture2D object, and call Texture2D.SetPixels to write the extracted section of pixels to the new texture object. Then, call GUI.Box() or GUI.Texture() or whatever else you want to use, but using the new Texture2D object you created instead of the old one.

My question would be. Do I gain something from having all my textures in a single, let's say, png file then do this to create subtextures to apply? Or it is better if the textures are split by the artist?