The current hardware might not be, but you could run this on a secondary chip similar in specs to the T2 or Secure Enclave. You could theoretically even have a second battery specifically for this purpose, but that would likely be cost-prohibitive.
The T2 chip is a derivative of the A10. It has similar power requirements to the SOC in modern iPhones.
They could probably use a cut down derivative of the W2 chip used in AirPods with the audio codec etc. removed. I’m guessing phone batteries reach a point where they’re still storing energy but can’t provide enough current to safely boot the whole phone. The BLE chip could sip on the remainder of the battery for a long time.
This would be excellent. I'm hesitant about the prospect of an ARM based macbook, but having both seems mighty compelling. As a lower power application CPU for light usage, or a full x86 with accelerators / co-processors when needed.