So such an alarm might be a netsplit/isolation warning, such as an in-sync connected client not getting a new block for an extremely long time and/or one or more blocks with extremely long timestamp delays. The time thresholds could be set to an example .00001 probability of taking so long. Such a warning would come up every few years on normal Bitcoin and be erased quickly when another block comes, but would stay present on a client on an isolated network until block finding returns to an expected rate.

It would only take one person smuggling a blockchain copy on a USB flash disk across the border to catch everybody up in censor-stan. Spending money (smuggling out transactions) would be a bit harder though; you'd have to get out a hibernated laptop with cached transactions or such.

It's hard to picture a scenario where a "freeze" would be needed or could be automated. However, payments you have received that have only been confirmed by extremely long block times could remain marked untrusted (indicating they may only exist on an isolated subnet far behind the current block count). Do you have a link to an explanation of "exponential climb"?