Last week I borrowed my friend's Linux laptop.
Open terminal; xmodmap "Dreymar's file".
That does the trick on anything not running Wayland, though the command takes like 4 seconds to execute.
It's about as easy as the instant 'setxbmap colemak'. Reverting it is the same 'setxkbmap us'.
I've already made a post on hassles Wayland causes.
OS X still requires going into settings (i.e. an admin password), making it equivalent to running an executable on Windows, like downloading bit.ly/clk or ryanheise.com/colemak.exe
For Windows I always use AutoHotKey or PKL. In most cases, people don't want you installing things on their stuff. Running something is okay though. Especially if you explain it's a script.
I agree when it comes to standards, for normies; Colemak, ColemakDH, Dvorak, Dvorak switching U and I, your momma's custom layout, and anything in between are all the same. Once you deviate slightly, you're a freakin weirdo.
But standards matter, when it comes to using some random setup and you have to negotiate your input method with a friend, admin or even your boss.
I know that back when I used Colemak I felt supremely confident I could use it at work regardless of the OS.
OS X is essentially the hardest to deal with because installing keylayout files or KarabinerElements are equivalently awful, permission-wise.
Back in the pure Colemak days, especially once Lion came out, it was just flipping a switch.
ColemakDH feels like working in a kitchen and having a personal knife that I always have to bring, whereas Colemak was just the nicest knife already available.
This knife is still better, but you can't take it everywhere.