No idea what a cage alteration would be in this context, but here we go. Not having a clue never stopped me having ideas.
Am I right in thinking that what you need is a few more bits between the texture islands? I only know this from the diffuse colour texture where colours run into each others when you get farther away because of the averaging this mipmap feature does? Now for a normal map, that certainly can have stupid results. Is that what you are talking about? If not, discard the rest of the post.
Solution 1: Tell the system to always use the maximum resolution texture for the normal maps. No idea how you do it for RW, but people do work with .dds and there is a flag "don't use mipmaps" in the DDS tools I came across (for Zusi, not RW).
Solution 2: Failing (2), create the mipmaps manually. For diffuse colour, there are guides out there. I would got for a texture that produces the default normal, i.e., just solid blue/violet, whatever, the colour that most of your image is made of. Taking that for all mipmap levels except the nearest one would at least stop the trouble, would it?
Solution 3: Get a headache. From the fact that you have one, I deduct that solutions 1 and 2 don't work. Solution 3 would be to shrink all the UV islands, both in the image and in the UV mapping. That would require a rather clever script to do. And assuming that you don't have the normal map in a monster resolution, you would get stupid rounding effects. So it might not be worth the effort.
Ok, now I am really curious what the problem is in fact.