Or, say I want to boot directly into a video game console emulator with Linux with 0 additional steps. How would I do that?
The cleaner way of doing this is through "inittab". Normally this starts a program called "getty" on the console which provides the login prompt etc. That's what you want to override. It will also restart your program every time it quits.
(Less normal options include "put your program in the initrd", which is viable if it's very small and does all the setup itself, or "write something that runs directly from the UEFI loader")
Also, you can use /etc/passwd to set the shell for every user. If you set this to a non-shell application, you essentially limit this user's session to only ever running that program. So a very lightweight, but networked system could set up sshd in innittab and then create a user with your single-user application set as the shell, and maybe an admin user that actually has access to a proper shell as a back-up.
There actually was a blog post I read not too long ago about a sysadmin who detailed the security challenges of running an open SSH server in which passwd set the shell to a game. There was a little bit more to it to get it completely secure. Unfortunately can't find it right now.
Instead of trying to detect hardware, network, and all, you could just hardcode everything. If you're working with a standardized set of hardware, that would most likely be simpler than anything else.
Note that pmOS does it to call an actual init system, but it could be done as an init in and of itself.