As far as I know, Iran is much too open an society to actually prevent its citizens from knowing anything in particular.

That's not to say it's a free society or that censorship doesn't exist there, just that it's not the sort of regime that is particularly good at it.

If I had to guess, Iranian expats would be a likely set of people to start up proxy servers for their family and friends back home.

Yes, which is why Signal is doing a disservice by telling people to announce their proxies on Twitter. The expats should just tell their friends and family, and tell them to pass the word on only to people they trust.

But this doesn't stop them from doing that. If you have an expat friend or family member with a proxy, use theirs, if not, check the latest tweet with the hashtag and use that.

That is precisely why I am suggesting using a CDN. Old school CDN that is. Back in the day, if you had Akamai, your site would just use one (or many) of their generic names. Nowadays you can use your own domain to front their network, but you don't have to. If Signal was using a few CDN's and cycled through many generic end-point names, then Iran would have to block all the CDN's which would be nearly the same as shutting off the internet. This would not have to be the default mode of Signal. It could be an option that the client suggests. "Hey, it appears we are blocked. Use alternate proxies?" Then cycle through many different CDN's using many generic end-point names. Some of the CDN's can also do layer 4 vips and not have to decrypt anything. They can just act as a TCP tunnel if need be, just costs more.

At some point, the easier option is for there to be a revolution or some sort of governmental change.

Easy to say when it is not your life or your families’ lives at risk.

True but not everyone is keen to experience the civil war that often accompanies such a change.

That's what lead to this mess in the first place!

Communication is key to both of those things.