How do I add a text editor to my (debian-based) initramfs?

Debian-based distributions including Ubuntu and Mint use initramfs-tools to generate their initramfs'es. To add an executable such as nano, place the following script in /etc/initramfs-tools/hooks/nano.sh:

#!/bin/sh    
# See `man initramfs-tools` for details

PREREQ=""
prereqs()
{
        echo "$PREREQ"
}
case $1 in
prereqs)
        prereqs
        exit 0
        ;;
esac
. /usr/share/initramfs-tools/hook-functions

# Begin real processing below this line
copy_exec /usr/bin/nano /usr/bin/nano

Make the script executable:

sudo chmod +x /etc/initramfs-tools/hooks/nano.sh

And rebuild your initramfs:

sudo update-initramfs -u

To add any other executable to the initramfs, change the last line of the script to poinnt to it. The copy_exec helper function automatically includes any necessary libraries.