@quantic, Yes, i had same issue. make a backup of /boot/grub/grub.cfg then edit it, adding linux and initrd lines.
@quantic, linux (loop)/isolinux/vmlinuz0 and initrd=(loop)/initrd0.img root=live:CDLABEL=CentOS-7-livecd-x86_64 rootfstype=auto ro rd.live.image rhgb rd.luks=0 rd.md=0 rd.dm=0
@quantic, inspect the ISO via mount -o loop /boot/grml/CentOS7iso.iso /mnt/somedir; && ls /mnt/somedir
@quantic, we have to get the kernel image and initrd path just right. i have tried so many paths... It would halt after 2 seconds, so initrd is likely bad.