Just an update: I've got a beta of BackupMenu nearly done; I've run into issue after issue with getting it working.
First, it was getting all the required files and permissions for the chroot. Then, busybox-tar decides to crash if a read error happens. Then, busybox-tar also won't add files to archives, only create archives.

I ended up using gnu-tar and gnu-ls, as the busybox equivalents wouldn't do. After finding the actual files, and their dependencies(and realizing gnu-ls resides on /opt/, so it had to be copied to the rootfs first...), I finally have it more or or less working.

Now, just some more fixes, and I'll have something better than before. Hopefully.