IIRC, Cyrus did not want to accept a not-bitcoin address because
this direction from theymos says it needs to be a bitcoin address.
I am not sure if theymos ever gave the okay to use an altcoin signed message, although he should allow major altcoin addresses to be used, perhaps specifying that litecoin, ether, ether classic, bitcoin cash, can be used, as usage of each of those coins is very widespread, and each of their cryptography is secure based on their standing within the market.