Could there potentially be more applications to this approach in terms of game anticheat measures, where you boot into a unified "tournament image" which is a minimal OS that only runs the game itself for competitive sessions, as an alternative to installing intrusive kernel anticheats?

I think you could even make this into a unified kernel image [1] and boot a single image directly from EFI.

[1] https://wiki.archlinux.org/title/Unified_kernel_image

Good idea. I used to have a similar "boot-to-DOSBox" setup which ran on the framebuffer, it was an easy way to have a little portable gaming USB drive.

We don't really need a whole entire OS to run doom.. I want to see a port where Doom IS the OS :P

Most minimal kernel configs are extensively tested. I've never had an issue booting from a custom kernel.

I had mostly 2 issues with custom kernels: graphics card not initialised properly (black screeen after boot, no reaction to keyboard) or X starting with missing (event ?) drivers - so no keyboard and mouse.

Similar concept, but NixOS + SM64: https://xeiaso.net/blog/super-bootable-64-2020-05-06

one file, declarative, can generate an image for a physical machine, single-command boot it in Qemu, etc.

Well, two files, if you package SM64 yourself. Still.