> I wonder how this changes by booting from UEFI (and not using any 'emulation mode')

It's pretty damn easy:

1. You don't write a bootloader since UEFI is that bootloader.

2. You write a portable executable which gets executed by the UEFI bootloader. This executable runs directly in long-mode without any nonsense.

3. You have to link against some different C-libraries, not the standard ones, but that's about it.

Here's an example: http://x86asm.net/articles/uefi-programming-first-steps/