Haha, I know that feeling: I wanna do it because I wanna do it! For the past 3 years I have been busy developing a Raspberry based, OS-free, C-free, Library-free, Forth based home-computer (like the ones in the 80s - fast booting and lots of fun to use). And there is NO logic behind it that other than: I like doing it.
Now for some power management:
On page 2.21 of the TRM of the Cortex-a53 processor ARM describes in detailed steps how to switch off an individual core. Problem being that you have to check the level at a pin (STANDBYWFI), pull another pin low, assert output-clamps etc. In other words: if Broadcom hasn't provided for that, than it cannot realistically been done.
In the same document it is also stated that WFE/WFI puts a core into a static state with only power drawn due to static leakage and a small amount of power drawn by the logic needed to be able to switch on the core. Which in my interpretation sounds like microwatts or even nanowatts.
Experimenting with switching off items like HDMI, USB-controller (if you like a bit of soldering), the power-led etc, etc.and managing frequencies has a bigger potential to save some real power. Buying a good power-supply might also be a good idea.
Just my thoughts. Have fun programming!
going from a 6502 on an Oric-1 to an ARMv8 is quite a big step...