you could use ftpiiu_everywhere but only if you replace files, and even that is not very nice.
ftpiiu doesn't set the file's permission correctly (owner/read/execute).
a user deleted entirely his vWii NAND with ftpiiu and put back someone else files (which is wrong, the keys and consoleID doesn't match anymore), but it didn't work due to bad permissions.
someone released a python script to set all vWii files in full permission (read/write/execute) if you really want to go that way.
But I don't recomment it.
The alternative I suggest is to launch a vWii wad manager injected into a WiiU Wii virtual console.
it will launch vWii mode and launch the wad manager with full NAND permission. From here you reinstall the proper vWii IOS to overwrite the Wii version you installed.
You can find a guide I wrote here :
https://gbatemp.net/posts/7687375
I don't know if you'll have any issue creating the forwarder channel.
I tried, and failed
it works fine for most users, I don't know why I can't do it myself.
if it fails, someone else might help you (but can't publicly share it here), or you'll have to manually create it instead of using a script.