It is as simple as installing the WAD, and then using AnyRegion Changer in the Homebrew Channel to change the video mode and console country settings.
When you are done, you can put it back.
If you prefer a virtual NAND, it is much easier, since you have a Japanese System Menu and a Japanese 'setting' for the System Menu, begin able to load anything Japanese with ease.
There is neek2o which allows for multiple virtual NANDs which can be switched on-the-fly - you can set up different region virtual NANDs.
The only thing this way is that the savegame and channel/game logs will not be stored on your Wii's NAND.
If you would prefer sneek/uneek instead of neek2o, you can set up one virtual NAND and create multiple 'setting.txt' files with one of the programs that comes with ModMii.
You can simply just go to the 'data' part of the System Menu folder on the virtual NAND and rename/replace the setting.txt file for the region/video mode you would temporarily like.
This way, you can have the same NAND and it is like the equivalent of using AnyRegion Changer, except you would manually replace existing preset 'setting.txt' files.
You can even create some windows batch files to replace the setting.txt files around for you, making 'changing the region' as easy as running a batch file.
I believe neek2o does not load on BootMii/Boot2, but I may be wrong about this. Sneek/Uneek work this way, and is extremely convenient.