I see that double tagging is available on the SRX300 series (flexible-vlan-tagging) by reading this docuemnt:
Configuring VLAN Tagging
But when I configured flexible-vlan-tagging on an aggregate interface, when I tried to configure 'vlan-tags inner outer' that was not an option. It was an option when I tried that on a physical interface. I assume it's just not an available feature on agg interfaces, but haven't been able to find any documentation on it. If anyone has a doc to point me to that would conclusively state the support of this feature on agg interfaces please let me know.