I finally received a Raspberry Pi 400:
Time to take it apart and see what's going on with the GPIO 5V!
Most reviewers talked about the top size of the PCB and pretty much ignored the bottom. Contrary as I am, of course my main interest of course is on the bottom, specifically with these two parts:
These are identical parts, presumably the load switch talked about. Two of them: one to switch 5V and the other to switch 3.3V. The one in the center definitely drives the GPIO 5V pins, the other one switches 3.3V but oddly is not connected to the GPIO 3.3V! I think it switches the SD card slot power. Protect the one GPIO power rail but not the other? I don't get it.
They are marked "0F=11R". It took some digging to find what exactly they are, but eventually I figured out that they are almost certainly Richtek RT9742 load switches. The connections match the datasheet, as does the "0F=" marking. Oddly, two variants are both marked "0F=": the 3A RT9742ANGJ5F and the 1A RT9742GGJ5. The only difference to tell them apart is that one of them is in the "TSOT-23-5" package and the other in the "TSOT-23-5 (FC)" package. But if you check the mechanical outline specification for these packages, they seem to be identical according to the spec. I don't know what the supposed difference is then.
So I decided to just test which one it is. I hooked up my electronic load, and increased load from 1A up in 0.1A steps. It tripped at 1.4A, so it's the RT9742GGJ5 1A version.
The datasheet has this info about the pass transistor: "Unlike a normal MOSFET, there is no parasitic body diode between drain and source of the MOSFET, the RT9742 prevents reverse current flow if VOUT is externally forced to a higher voltage than VIN when the chip is disabled." So indeed no hope of turning the Pi 400 on from GPIO power.
Once the Pi is on, it may be possible to preserve power long enough for shutdown though. The datasheet says: "If VOUT is greater than VIN, current will flow from VOUT to VIN since the MOSFET is bidirectional when on." So UPS functionality is still possible.