I believe you could accomplish this by changing your wii's region with anyregionchanger, then getting whatever japanese VC titles you wanted.
If you wanted to revert to whatever region you're at after the download, you could use wad creator to extract what you downloaded, then region free patch or whatever region you prefer patch the wad. Then you could install the region free or whatever region wad to your wii once you change it back. I don't think compatibility using this method is perfect, and it can take a lot of work. It can also be very risky if you don't know what you're doing.