Like I said before, there is no site that want to sign message just to prove they have enough bankroll to cover all the withdrawals. I know that you need the proof so people will stay calm that their withdrawal will be process but it does not mean site need to give you that because it is the privacy for them. May be if they send it to other address and sign it from there might be possible but to sign from the main address I do not think it will work that way
There is a difference case here, Zoomdice said that they will sign a message months ago. This is why some people re-asking about it, the reason behind the sign a message of this site was because they took so long time for small withdrawal. The issue made people worry to play, that's why they ask for proof of solvency by signing a message and Zoomdice agreed with it.