This brings back memories - I owned a large web hosting company and we had thousands of machines. When hardware issues came up, or machines wouldn't boot, using this method was our first line of defense - we'd boot the machine from a burned copy of "Recovery Is Possible" which was an all-in-one Linux distro for recovery, then mount the partitions and chroot in to figure out what is going on - or use rsync to migrate data off as needed.
Just looked and it looks like "Recovery Is Possible" hasn't been updated in a dozen years which dates my story, but I fondly remember overnight phone calls from panicked new sysadmins and telling them to be calm and "RIP it and get chrooted in" and then waking up to help them troubleshoot.
System Rescue is what i use these days, but to be honest i can’t recall using it in the last couple of year. Virtualization and containers have taken over.
System Rescue used to be Gentoo based and then switched to Arch. You can install both Arch and Gentoo with SR and obviously you can install Arch with a Gentoo install CD and vice versa.
SR also has some rather handy SAM database editing facilities. Mount Windows at /mnt and then enable and reset the Administrator password. Jolly handy for getting super duper user access on a Windows box.
Its been a while since I installed Gentoo but you can probably quite easily add more stuff to your Gentoo install CD. I don't know if Gentoo have added a script to do all the bind mounts but "arch-chroot /bin/bash" is very convenient. I used to forget about /sys.
Just looked and it looks like "Recovery Is Possible" hasn't been updated in a dozen years which dates my story, but I fondly remember overnight phone calls from panicked new sysadmins and telling them to be calm and "RIP it and get chrooted in" and then waking up to help them troubleshoot.