I know that there are plans to eventually add a way to download legal wads (ie. forwarder channels) from a database and install them (might even be possible to only download banner and icon, and wad manager would create the forwarder dol on the fly based on the folder you select to forward to).
Don't hold your breath for this feature, it is at the end of a long list of ideas the dev has to add to the new wad manager (i wont name names, but u can probably guess which dev I am referring to, but I don't want to say too much).
The point is... it is definitely possible to download pirated wiiware wads and install them on the fly... but a database would probably have to maintained and linked to the wadmanager... and every wad could be a possible banner brick as there is no way we could quickly test them all... it still sounds very shady and I doubt it will ever be done (in fact, I hope it doesn't, if u want to pirate, just download the wads using your pc). I personally don't want wad manager becoming known as a tool for piracy, its not (even though it can already be used to serve this purpose... but so can a lot of apps that aren't designed for piracy... like the HBC)
Also, I don't think it will work if the file is hosted on mediafire (it would probably have to be a service like filetrip that has a direct download URL that doesn't change every X minutes like mediafire)