LOL, that didn't take long. At the time this security measure was being discussed, we were debating whether the default should be block or allow. There always seem to be an outlier.
Frankly, updating a remote router's firmware seems pretty risky, just in general. Unless you have a *very* desperate need for a new feature or bug fix, this is something better left for when you are present at the remote site. And a *beta* ta-boot.
Assuming you accept the risk and intend to maintain the existing nvram settings (something I don't normally recommend either), I suppose you could add some firewall rules to the firewall-start script that precede the blocking.
Code:
SCRIPTS_DIR="/jffs/scripts"
FIREWALL_SCRIPT="$SCRIPTS_DIR/firewall-start"
mkdir -p $SCRIPTS_DIR
cat << "EOF" > $FIREWALL_SCRIPT
#!/bin/sh
iptables -I INPUT -i tun1+ -j ACCEPT
iptables -I FORWARD -i tun1+ -j ACCEPT
EOF
chmod +x $FIREWALL_SCRIPT
This assumes, of course, you have jffs and custom scripts enabled on the Administration->System page.
Once you have access to the OpenVPN client again, you can change the "Inbound Firewall" option to "Allow" and get rid of the firewall script. Or perhaps leave it there for future firmware updates.