It is possible. Here's some tips:
- All the DLC songs are downloadable directly from nintendo using any web browser. Figuring out the URLs isn't hard, if you look at the URLs for the IOS updates etc.
- Naturally they're encrypted, the decryption key is in the ticket (like any normal title)
- Getting a valid ticket is easy, just download a free song and do a nand dump. All the GHWT DLC content uses the same title hence all songs use the same key (with different IVs).
- The title_mask in the ticket has to be tweaked so the wii thinks you are allowed to access all the DLC.
- Modifying the ticket means it will have to be fake/trucha-signed.
- Figure out how to install a few songs at a time onto the wii, or be extra clever and work out how to encrypt them for direct storage on the SD card (still requires the modified ticket to be installed on the wii).
Note that I never mentioned the save files at all. That's because they've got nothing to do with it.