Quite mind bending, well done for finding that. Even Litecoin devs are now realising that network security is everything.
My first thought is that we need to enforce miner version(s), but obviously that is difficult (impossible?) to implement and would have to be centralised and is against the general open ethic of the (Satoshi coin) protocol.
The second option is to be open and monitor for “unfair” allocation of funds to particular miners. Again, not perfect, but is my preferred option, as it does not effect the protocol.
On the other hand, Feathercoin does seem to be maintaining a sufficient mining base to make such an attack technically difficult, it might even have to be run by a pool. This could mean community pressure might work to restrict this being implemented.
Maybe, we need a a traffic light system to mark when we are more liable to attack (awaiting bug fix, low Hash rate, known attacker identified). Pools could be “Feathercoin Audited” for using standard / up to date code version, and listed on the forum.