Not easy, but not impossible, because it turns out the PowerPC processor inside the Nintendo Wii is close enough to the PowerPC chips Apple used in its Power Macintosh G3 machines for this to work.
Pierre Dandumont explains how they managed to get Mac OS 9.2 running on a Wii on their website (courtesy of Google Translate) and as clever as the hack is, there’s layers of complexity that result in the operating system running less than perfectly.