Trying to boot an ReaR-created ISO from the hard drive

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I've created an integrated ISO (both backup and recovery) of my RHEL 7 system that is a virtual machine. What I am trying to do next is to force the system to boot from that ISO and proceed with recovery. Burning that ISO on media or network install are not options in this case.
I understand the way to proceed is to edit the /etc/grub.d/40_custom file with the custom menuentry for the rescue ISO I created, but how would the contents look like?
The ISO itself has two directories, backup and isolinux. The second one looked more promising for finding the kernel and its associated initrd. isolinux directory doesn't have vmlinuz nor initrd.gz or similar, it has the "kernel" as executable and initrd.cgz as the closest resemblance to an initrd.
Had anyone booted from an ISO created by ReaR without burning it on media or doing network install but from device's own HDD? And how would the 40_custom file look like in your case?
I've edited 40_custom on my own approximations and I goot three errors:
error:filename expected
error:no server specified
error:you need to load the kernel first