I'm not sure I'd count on it to evade the spooks as twelve chars is twelve chars but here you go:
mkdir -p /root/.bin/ # make a directory to store the shell script/sh
cat <<- 'EOF' > /root/.bin/rand-hostname
#!/bin/sh
# shellcheck shell=sh
RANDOM_HOSTNAME="$(tr -dc 'a-z0-9' < /dev/urandom | head -c 12)"
uci set system.@system[0].hostname="${RANDOM_HOSTNAME}"
uci commit system
/etc/init.d/system restart
logger -t hostname "Host name changed to ${RANDOM_HOSTNAME} [✔]"
/etc/init.d/network restart
exit 0
EOF
chmod +x /root/.bin/rand-hostname # make the sh executable
Always execute this before connecting to WAN/WWLAN or via Repeater otherwise the device's real host name will be transmitted to the upstream network provider before you change it:
/root/.bin/rand-hostname # run it
logread -e hostname # check the device logs the hostname was changed
uci show system.@system[0].hostname # show the current hostname in effect
Pro Tip
Update your .profile path to include /root/.bin so you can run it right after logging in via SSH by rand-hostname. See related below.