pfctl to add rules at runtime WITHOUT editing /etc/pf.conf?
I solved this myself.

cat /etc/pf.conf my_rules.conf | sudo /sbin/pfctl -Ef -


Where my_rules.conf contains our own rules, these get concatenated to the pf.conf rules but take precedence due to them being appended after pf.conf in the cat command.

Also note we use pfctl -Ef -  the - forces pfctl to read from stdin

We can take this solution a step further and load our own rules from stdin by using this:

echo "block out all" | cat /etc/pf.conf - | sudo /sbin/pfctl -Ef -