This is a problem I have come across also. You are right that to receive an untagged frame on ingress and to have that frame double tagged on egress is not possible on the SRX. However, do you have two free ports on the SRXs? I know it is messy but you could recive the untagged frames on a standard 802.1q access port and egress the frame on a standard 802.1q trunk port, then use a cable to loop it back into the SRX on the 802.1ad access port to allow it to egress on a 802.1ad trunk port with the two tags. If you are using spanning tree you will need to disable it on one of the loop ports. Not the cleanest solution, I know.
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