I thought that the free cable that you send off to MS would only work with your own XBoxes and no other, hence the need to give them the serial numbers of both consoles. Is the cable you're sending around the one ordered free from MS or is it a different cable and thus the DRM issue won't apply?
All cables should do the same thing. the whole DRM issue is solved by transferring the licenses over to the new machine - you should only need to do this once - checkout the guide on the offical xbox 360 site.