iptables rule to allow all outbound locally originating traffic?

You need two rules to do that:

iptables -I OUTPUT -o eth0 -d 0.0.0.0/0 -j ACCEPT
iptables -I INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT

Some notes.

• Preexisting rules that you may have may do this already, but look different.
• This uses -I to force these rules to be first. iptables rules are evaluated top down.
• The -o and -i flags mean "out" and "in" respectively. Replace eth0 with the proper ethernet interface name.