Just do a hash of the message text and it works if you cut and paste it too. It will break if you modify it but at least they can track it back to you, identify the source of your message and then track it on.
For this system to work they will need access to 1 message in the chain - usually someone who infiltered the group or someone who was offended by the message and reported it. Once you have that you can track the chain to its source and get a warrant for that person and then repeat the process after examining their phone to track their source.
It seems like a workable solution that does not break encryption - not that I support it.