My thought as to what I would like, and consider as the ideal situaton:
When a message is sent (i.e. goes through the groups.io system) as a special message (however this was triggered) a tag (or other marker) should added saying #This_Was_Sent_As_Special_Message, or word(s) to that effect. If a message is sent as a not special message with that tag, then it should be removed.
If the trigger to a message being sent as a special message is a tag (#Send_This_As_Special_Message), then that tag should be removed as it goes through the system (so that tag will never be on a received message), and replaced by the #This_Was_Sent_As_Special_Message one.
Critical element of this is difference between inwards (to group.io processing) and outwards (from groups.io) tagging; and the removal of the outward tag if isn't correct on a reply/forwarded message.