you want to remove pieces of the mesh - and leave the uv co-ords in place?
the co-ords are vertex dependant soonce you remove say, a triangle, the information for that triangle will be removed. You an create a new triangle and put it in there but it's going to need tweaking. If you are importing uv's then it's looking for names of vertexs not the general shape of the model.
I don't understand why you are trying to remove part of the mesh and keep the uvw co-ords. Are you rendering out some normal or shadow maps? If you just backup the file, delete the mesh, render the map out and then open it up the original file, etc.
Maybe you can post some screenshots of what you are trying to do?
My english sucks so I don't know how exactly write what I mean. Ok, I will tell it easly:
1) I have something like that on mesh and I want to delete a part of the mesh in red elipse...
[Blocked Image: http://img231.imageshack.us/img231/4826/66835454io2.jpg%5D]
2) With help of my brother now I know how to delete it and don't have problems with UVW's...
I had to add new modificator "Edit Poly" upper the "Unwrap UVW" and then delete this part of the mesh using option 'Remove' becose if I weld some verts I will change UVW mapping. Thats all 
What seir said - add a Edit Poly modifier to the stack after the UVW Modifier and make sure you have "Preserve UV" turned on.
Or, just apply UV's (collapse the stack), delete what you want, and recreate your uvw modifier.
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