...Or completely replace the innards with a Raspberry Pi Zero W. (Not sure if it’s actually possible.)

Various pi boards have been retrofitted into laptops but a Pi Zero in particular is a downgrade in performance from just about any relatively recent laptop/Chromebook. As some of the sibling comments mention, there is more involved with resetting than an accidental non-action during boot.

Even better you can switch to coreboot and load Linux, there's even a Chromebook-specific distro, https://galliumos.org/