Thanks for giving it a look! You're certainly right about UEFI. I figured it would be easier and informative enough in the long run to just write a simple MBR bootloader instead of an EFI-format one.
It surely was fun, and I personally liked the way you detailed and commented the code, but that is not a bootloader, it is a bootload, i.e. something that is loaded at boot, executes some instructions and doesn't actually load anything.
In the good ol' days there were tens of similar bootsectors (to be used on non bootable floppies) that could display mesages or draw ASCII art.
If anyone wants to play with actual MBR bootloaders, a basic MBR is by Peter Anvin (and Andrea Mazzoleni) in makebootfat:
Either way, it was definitely fun.