I believe the solution to this problem is to ban ROCOF protection, and the related phase shift protection, and instead instruct a few big energy producers to transmit a gold code on top of the 50 Hz AC, bandlimited to 48-52Hz and power limited to 0.01% of the system power. Transmitting that code would be easy (cheap) for anyone who does DC/AC conversion with solid state electronics, so that's normally solar, wind farms, and long distance undersea transmission lines.

That gold code could be received and decoded anywhere on the network. If power islanding occurs, embedded generation will detect the loss of the gold code (since they are no longer connected to the generator injecting the code), and cease supply.

The only disadvantage is it introduces a security vulnerability by design: Anyone could transmit the gold code from their house, effectively disabling islanding protection in their neighbourhood. If power islanding were to occur, and if there was sufficient embedded generation to keep a stable power island, grid hardware could be destroyed through overvoltage, overheating, and circuits closing without frequency synchronisation. I think it's a worthwhile tradeoff though - damage will be localized and minimal, and a very unusual set of circumstances have to happen outside the attackers control for the attack to do damage.