Here's a tool I did program to investigate the Hardware State Region, It displays all the 64 bytes in the hardware state, you can see whenever you re-run the tool the same data until you reboot. (try also to reset the cart in the browser, it will still show the same data)
After reverse engineering I found that the addresses 0x5F40 -> 0x5F43 does something specials which is music corrupting as shown in this topic: https://www.lexaloffle.com/bbs/?tid=3561
And addresses 0x5F44 -> 0x5F7F are not used at all, and can be used to keep data between cart resets or cart changing, but clears on reboot.
I'm not sure if they are used in the other platforms like rpi or chip, nor if they will be used in future updates.