[Renate]

There's a long route from physical buttons to having an app do something. Things can be changed at many points along the way.

Button
Image dtb
Linux keylayout
Onyx keypad.json mapping
App dispatchKeyEvent

Are we talking about the Leaf? Do you own one?

Although it's the more obscure way to change things, if I had one, I'd change it in the dtb.
Why? Because even if it's trickier to binary modify the dtb on the kernel, it's not verified or error corrected. You don't even have to be rooted.

Edit: Also, many reading apps respond to VolUp/VolDn as page commands. After all, they are designed to work with phones too.