They already know people who are trying to access signal without a proxy, so I don't think this would make a significant difference. Also note that from the Signal Blog post above:
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The Signal client establishes a normal TLS connection with the proxy, and the proxy simply forwards any bytes it receives to the actual Signal service. Any non-Signal traffic is blocked. Additionally, the Signal client still negotiates its standard TLS connection with the Signal endpoints through the tunnel.
This means that in addition to the end-to-end encryption that protects everything in Signal, all traffic remains opaque to the proxy operator.
It doesn't seem to be the same situation with tor exit nodes, where your node is automatically on the system. Here, it looks like people have to actively use your proxy; it tells people who run a proxy to share a URL with their friends.
And that brings the difficulty of letting your proxy be known to legitimate interested people if your iranian social presence is non-existent. I ran a Tor node (not an exit one) in Germany back in the days (it was to help iranian people).
A regime that has survived 40 years facing constant adversary and the majority of time under sanctions should be competent enough at internal security.
And the people that are protesting and hurting right now are not the most tech savvy one - so expect a lot of naivete about opsec. I doubt that the majority of them even know signal exists.
