I have read many posts about screen resolutions for Ubuntu as VirtualBox guest. May be I failed to read the good one... So.
I "just" want to make more screen resolutions available WITHOUT installing/modifying anything requiring superuser access in the Ubuntu guest VM (then WITHOUT guest additions). This requirement is a mandatory security issue for my project.
I am quite confident that it must be somehow possible, because setting "GRUB_GFXMODE=1280x1024" in /etc/default/grub uses 1280x1024 in grub boot menu only. This shows that the Virtual Box virtual graphic card does support it.
PS: please don't reply anything involving guest additions..