The best method we came up with, which would make it harder but not impossible to claim would probably suffice and it would be achievable with a soft fork and the exchanges listing BTC2 wouldn't even need to update their wallets.

It would take some careful planning or a lot of trial and error by exchanges to know which Bitcoin addresses could be used to claim BTC2, because most of their addresses probably aren't old enough.

Another plus side to stopping claims eventually is that then the bigger exchanges wouldn't need to worry about rewarding BTC2 balances of over a year ago to their users and thus it would be much easier for them to list BTC2.