I did a quick check with inkscape: when I group two objects, that are in different so called "layers", the group will then become part of the upper "layer". "layer" information of the objects themselves is lost by grouping them. This is an irreversible action. Exploding the group will put all the members into the "layer" of the group. This can be easily watched in the XML-Editor window of inkscape: "layers" are indeed groups with special attributs, but members do not reference these "layers" by special attributes at all.
As a consequence, as of Inkscape 0.48.2 r9819, there is NO way of transparently/reversibly mapping *two distinct hierarchies* like blocks and layers of QCAD+DXF to inkscape+SVG.