Something like a secondary bootloader. Is that possible?
Yes it's possible. As you mentioned, since the boot chain up to bootloader stage is closed source and binaries are signed with OEM's private keys, it's not possible to replace or modify the bootloader. However if bootloader is unlockable, it's possible to boot a custom kernel (your own OS). But the first thing is that you need to follow the standard format of boot.img (i.e. fixed offsets and file formats for kernel binary, ramdisk, DTB etc.) as specified in Android (AOSP).
Secondly building a "Hello World" kernel (or even a bootloader) for a PC is rather simple because many of the buses and the hardware attached to them are already recognized by BIOS/UEFI. However mostly Android devices are ARM based, so they don't have ACPI like in BIOS/UEFI. Instead hardware mapping is provided in the form of Device Tree which is provided by the SoC vendor. The Android/Linux kernel source provided by OEM also includes drivers for all hardware components. So you need to include the Device Tree Blob (DTB) and relevant parts from kernel source with your custom kernel.
Bootloader itself is built with the DTB to use hardware components like eMMC/UFS, display, USB etc. to be able to flash partitions, show boot splash screen, connect to PC etc. For instance, splash_region or freamebuffer in DTB specifies memory addresses of display hardware, which is required to print e.g. Hello World on screen.
Thirdly many of the kernel interfaces are proprietary (not documented) and they don't work with standard Linux ioctls/syscalls. In order to fully make use of hardware features (camera, gps, modem an so on) you need proprietary binary blobs (HALs) in userspace, again provided by device OEM. Android framework uses HIDL to communicate with HALs, so you also need to rely on that.
So what you come up with is not a purely “other than Android†OS. For a large part you have to share the same codebase, specifications and resources that run in Android OS, whether it's kernel or userspace.
For more details see: