it's already supported. i think it always has been possible to move your uvs outside like you want and since a while it's also possible to let the texture repeat outside (this can be set in the view properties panel -> repeat image).
this would mostly be useful in situations where fitting your entire model within one space is not practical due to restrictions in map sizes (largest being 4096^2) in some apps and with many video cards. sometimes to get the detail desired in a map which would require map sizes larger than 4096^2, such as a normal map, one must divide the detail among UV islands in different spaces with multiple maps of 4096^2.
perhaps my request isn't explained in enough detail. I would like to be able to have UV islands assigned to different textures applied in different spaces/tiles in the UV Editor. I would also need to be able to export an .obj of that model carrying with it the UV spatial information.
ZMapper in ZBrush supports the creation of Normal Maps of the largest size supported by one's video card--up to 8000^2. In my case, my card's limit is 4096^2, so if I have details for an entire model that won't be recreated well in a normal map at that resolution, I have to split up the model's UVs into different spaces and have ZBrush create the maps per UV space/tile/group. many modeling apps support this and the import/export of this UV information.