I have a disk image that is meant to be flashed and run as a live-USB.
Is it possible to boot from this image in Grub in a similar way to booting from an ISO with chainloader+1 ?
How would the menu entry in grub look, for example?
Chainloader can't do:
https://www.gnu.org/software/grub/manual/grub/html_node/Chain_002dloading.html
Chainload windows.img in grub2
Chainloader can load another bootloader from a boot sector or a partition. It gives the control to that bootloader to continue the boot process. So the answer is no, Chainloader cannot load the image directly