Hi Dave,
I have never used an ISW switch. The documentation suggests that a HYBRID port type is used for Q-in-Q. The C-Tag or inner tag (client tag) is used to distinguish client VLANs, and transported inside a Q-in-Q tunnel. The S-Tag or outer tag (service tag) is used to distinguish different transport tunnels. You would probably configure a hybrid port to place every inbound frame into a service VLAN, and send frames from the service VLAN without the S-Tag out of that port. To transport the Q-in-Q tagged frames you use trunk ports that allow the service VLANs.
I cannot tell you how to configure this on an ISW, sorry. 😞
Erik