Problem with trim edges are that they don't actually cut away anything. Because they can be arbitrary in shape and complexity and a general nurbs surface can't, they just define an mathematical region where the surface will not be rendered (usually the whole surface is still stored in memory, unless 'shrink surface' is checked and then it will cut to the nearest isoparm). So I guess that if you made your isoparms line up with the trim edge you could theoretically rebuild it or detach it someway, but how hard that would be I do not know.