Hello!
It only took 11 years but I recently updated my fork of vcromclaim to automatically calculate the encryption key and decrypt/decompress most games now.
I can clear up some things:
Some WC NG games are raw (at least two different file formats exist)
Some (bin.z) are compressed with zlib)
Some (bin.zx) are compressed with LZMA and encrypted with AES
The AES key is calculated using the content of banner.bin AND the first 20 bytes of the compressed ROM archive. (The actual encrypted data starts after that.) (It's also using some other data for input, that is found in other app files, that seems to be the same for every game.) So if swapping encrypted ROMs you would need to decrypt and re-encrypt the ROMs with the correct key. Replacing/changing banner.bin would require the same. I would not be surprised if the emulator does additional checks to make sure the game hasn't been tampered with though, so this might not be enough.
All encryption used by original NG hardware has been decrypted in the Wii versions, probably to save performance and memory on the Wii. Thus some games extracted from Wii have to be encrypted with the original algorithms to be playable in MAME. Also most likely the VC emulator will NOT accept games with their original encryption.
Some games also have different encodings and compression of the graphics, they probably would not fit into RAM if they didn't.
Note that the Wii does NOT have a lot of RAM, and the entire game MUST fit in RAM. Hence I guess some of the largest will simply never be possible to inject.
As you see, there are plenty of different formats. I don't know how flexible the VC emulator is, whether each game is bundled with a customized emulator or whether they will happily accept different file formats.
Let me know if you wonder anything.
If there is some NG game that vcromclaim can't export, please create a bug.
DISCLAIMER: I won't touch anything illegal, I'm doing this for the sake of preservation only.