> Making a simple tweak like setting caps lock to an extra ctrl is a whole side quest on Linux

I've remapped CapsLock to Esc on both macOS and Linux/GNOME. It's just a checkbox/combobox in a settings in both cases. What is 'a whole side quest' you are talking about?

Part of the fun with that is there's about fifty different places you could do it, and the only one that will really affect everything is in the kernel (IIRC), anything else will affect the program, window manager, terminal, etc but not necessarily anything else. And some programs bypass everything and read scancodes anyway.

Whereas in Mac OS X it's a simple configurable option (a bit hidden, System Settings, Keyboard -> Keyboard Shortcuts -> Modifier Keys).

And if you do it in the kernel, then any other users on the machine are stuck with your choice. The MacOS setting is per-user, but affects very nearly everything, except the recovery environment and I guess the login screen, for good reasons.

It makes me wonder, how exactly would one go about implementing Linux remapping that works on everything on a per-user basis? Is there a way to hook into the USB HID implementation and change what it reports to X, Wayland, etc and flip it on/off based on user preferences?

You'd likely need to have the kernel be able to switch "keymaps" at anytime (and this is likely to be privileged and need to be done by root).

I know there was a way to load different keyboard layouts, but the problem with those mechanisms is they often cannot remap modifier keys, of which capslock is a special case of.