Would you consider splitting the download of Classic and HD resources. If you have only RoC/TFT keys on your account or Classic is set in the settings, do not download HD resources. And start downloading when switching to HD
As I remember, when there was a CASC release, there was also a Cataclysm release. And one of the innovations, it was possible to partially download the client to start the game. so technically it should be possible.