Raspberry Pi 2 GPIO levels after shutdown

There are now many tutorials, Instructables posts, and even devices which help with adding a physical shutdown button to the Raspberry Pi. I want to build something that will completely and utterly shut-the-chuff-off i.e. switch the mains off. I aim to use a PICAXE microcontroller to handle this shutting off via a relay, but the PICAXE needs to know when it is safe to proceed.
I used a handheld oscilloscope to measure the voltage levels of the GPIO pins while the Pi was running, versus it being shutdown with the power still connected. The ONLY pin with a significant change in voltage is GPIO14 (TXD0) on physical pin 8. It drops from around 3.4 V to around 1.3 mV, so I’m hoping that I can use this to indicate to the PICAXE that the Pi has shutdown, or is in the process of shutting down and programme a delay of, say 10 seconds.
This could also work as a fail-safe such that only IF the level is low for this entire duration will it proceed with the switch-off.

4pin Raspberry Pi GPIO pin-out from Farnell, with annotations.

40-pin GPIO of the Raspberry Pi 2.

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