For some other moderators, signed messages from altcoin addresses can be used as proof of ownership if you get in unexpeted trouble that needs to freeze the account, so you can un-lock it.
As far as forum-specific issues go, it would be far better to assume that
only signed BTC addresses are accepted. There's no need to roll the dice on signed altcoin messages as there will always be a chance that they won't be honored. This is a Bitcoin forum after all.
it is not just about this being a "Bitcoin forum", the problem is the lack of a trusted wallet to do the signing and verifying of signatures and also whether the person verifying has that wallet installed. otherwise the processes are the same. bitcoin uses secp256k1 curve while ethereum uses secp256r1 curve with only different constants but same size signature and same EC calculations.