When you move a channel to SD, the Wii encrypts it with something called the "SD Key" which is unique to each Wii. This prevents users from copying from games from their SD card to their friend's SD card, or otherwise sharing the SD card copy of the game.
Creating a WAD Installer that would install WAD's to the SD card (either on the PC or the Wii) is definitely possible. It's just that no-one has taken the time (or shared their work) to reverse-engineer the SD card content.bin/data.bin files. In theory it should be possible to decrypt the content.bin into a generic format, that could then be re-encrypted using another Wii's SD Key and then work on that Wii. As we all know, the Wii-specific keys can be dumped using BootMii or Xyzzy. It's just a matter of reverse engineering the files and figuring out how to decrypt/encrypt them in the same manner that the Wii does.
In short, it's possible, it's just that no-one's done it yet.
Edit: Also, as r9delta mentions you still need the tickets on the NAND. So the PC version of an SD-card WAD Installer isn't very likely. It should be possible (with a lot of work and reverse engineering) to modify WAD Manager 1.5 though so it installs WADs directly to the SD card.
