I'd use bank accounts, PayPal or money transfer with small transactions and a one time pad to signal. They'll never ban bank accounts and it'd be hard to find signals amongst the noise. Or you could just use pgp and paste it into whatever app you want. Pandora's box has been opened, its remarkably naive to try to ban secrets at the same time as hoarding an unprecedented number of signals.

Obviously you can't intercept signals from someone using outlawed encryption, a one time pad or no direct messages. I'm not sure the stated goal (stop evil terrorists) is the real goal though - reading almost all communications and selective leaks is just such a useful tool for things like subverting democracy, throwing elections and controlling politicians.

> I guess you still need a central server to let clients find each other in the first place.

Would you? Couldn't you have a list of servers stored in localstorage, and bake the initial list in a bunch of seed copies?

Alternatively, use pastebin or imgur or something like that for your "central store" to pull from initially, then store everything in localstorage after.