>>9
Wouldn't a Gentoo prefix work? https://wiki.gentoo.org/wiki/Project:Prefix
Alternatively, you could just create a dir and extract a stage3 there and chroot into it. Just don't partition the disk or install a kernel/bootloader. You could also use pacstrap (https://github.com/archlinux/arch-install-scripts) to do the same for an Arch chroot. Once you got your dir set up, you can just chroot into it:
# copied from https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Base#Mounting_the_necessary_filesystems
mount --types proc /proc /mnt/new_linux/proc
mount --rbind /sys /mnt/new_linux/sys
mount --make-rslave /mnt/new_linux/sys
mount --rbind /dev /mnt/new_linux/dev
mount --make-rslave /mnt/new_linux/dev
mount --bind /run /mnt/new_linux/run
mount --make-slave /mnt/new_linux/run
chroot /mnt/new_linux /bin/sh
I think you could even create a Glibc chroot in a musl distro such as Alpine Linux.