I tried to unpack an ISO of Ubuntu to USB stick without writing it (i can’t use superuser utilities like dd or Rufus). Then i set my USB stick first in UEFI boot priority. I got working GRUB, but when i tried to run the system in live i got this error:
Initramfs unpacking failed: Decoding failed; Unable to find a medium container a live file system Attempt interactive netboot from a URL?
Maybe there is a way to run it from grub shell?
After some tries i found out i can repack Arch (it was archlinux-2020.10.01-x86_64 image) and some UEFIs will be able to run it’s grub. The Arch itself won’t run but it will startup an emergency shell (which seems to not be a thing in Ubuntu). This shell have mount and dd utilities, so it is possible to create a bootable device afterwards. And you don’t need superuser rights to just repack an ISO to USB stick (in fact, it was done with SD-card on non-root Android smartphone). It worked on 2 out of 3 laptops i tried.
So i repacked an arch.iso into SD-card and also placed ubuntu.iso into it. In Arch’s emergency shell i wrote ubuntu.iso into the USB stick by dd. And then i was finally able to run it.