In case of attacks, every site and every merchant should stop doing bussiness in that currency, also all the trade of the currencies on the market should immediately be halted. If network starts showing serious trouble, like in the case of FTC, everything trading in this currency should stop. If there come to unwilling block changes, the wallets should be restarted to pre-attacks balances. There should be standard set of "what to do" in case of 51% attacks. If a coin cannot apply to these standards (because of bad development team for example), that would impact its significance (aka label it shit coin in some cases). Possible legal prosecution of doers in countries where it is possible.
Games around 51% attacks could be moved to alt markets, where 51% attacks could stay legal, if that was what everyone would want.
Just my opinion of possible solution.
Yes thats a great idea to have a contingency such as this, great suggestion