Would AmigaOS 4.1 theoretically run on this?

[Piru]

Is PPC standardized in a way that full compatibility IS possible without chipset drivers for every single board?

no

There however is quite large base of fairly* standardized systems: PPC Macs.

*) If you rule out the exotic older systems the rest are using standard components, USB for keyboard/mouse etc.

[Piru]

have fancy cases that are horrible to open and void your warranty if you were to try.

Where did you get that idea from? You certainly won't void your warranty. Apple even has great documentation how to say for example upgrade the memory: https://support.apple.com/kb/ht1270

Even the manual that comes with Apple computers tells you how to open the machine up and replace the memory or HDD. For example for MacBook Pro 13": http://manuals.info.apple.com/en_US/macbook_pro_13inch_early2011.pdf (Chapter 3)

[Piru]

Not sure how the SAM chips fit into the G3/G4 categories.

They're G3.