Is it possible to go the other way? Make a PowerMac into a Nintendo Wii?

Not exactly what you're asking about, but related and from the same time period. The original Xbox 360 dev kit was a PowerMac G5.

https://apple.fandom.com/wiki/Alpha_Xenon_Development_Kit

Soooo... can you make a Wii out of an Xbox 360? :D

Xbox Wii60

The Nintendo Wii notably has two processors that run different operating systems, so you might need some emulation or hardware modifications to do so

The GPU would also be a problem. I don't think the Wii/Gamecube's GPU block really corresponds to any ATI/AMD card.

What you could probably do is take a Wii emulator like Dolphin and virtualize the CPU. Basically combine Dolphin and KVM_PR, getting ~native CPU performance while still needing to do 3D translation and simulate the IOS side of the Wii.

(Wii games run with the CPU in the supervisor state IIRC, so you can't just run them as processes under an OS, you'd either need a proper hypervisor or to catch traps caused by running supervisor instructions in "problem state", a la KVM_PR.)

but why? :)

There's no point.

Apart from exporting experience to future endeavours with more "point".

And "fun", relevant as other are insisting noticing, is another thing you build and will return in the future.

Doing things that are fun is the point

The author literally said that he did it for fun and to learn, at the beginning of the post