Sorry for the late reply, had to work through a few iterations... But it works now ^^.

Turns out the issue was not really in the grub and or initrd/vmlinuz...

I worked through your LiveCD guide, but did not en up with a working iso in the end :(. However, it did remind me of this guide: https://help.ubuntu.com/community/MakeALiveCD/DVD/BootableFl... on which distroshare is based. Running through the original guide resulted in a bootable iso. After checking, none of my previous homegrown isos were ever bootable on their own :O!. Apparently the level of black-magic in Unetbootin was greater then I had anticipated.

Best part of all, that iso is created on a non-EFI machine, and boots in EFI-mode just fine due to the grub-install --target=x86_64-efi on the bootstick. Everything else is just loaded over the original EFI-modules apparently. So no need to completely revamp all of my old code, just need to take a looksy into the iso generation part (used mkisofs instead of grub-mkrescue in my old scripts because of >4GB fat32 issues).

Thanks for all the help and interest man. It really kept me (mostly) sane ;)!