Our currently cold storage doesn't support signing message.
Any one tell us how to sign a message with a multi signature address. with 4 public keys and the amount of signatures required to release the coins is 3?
It's a pain in the ass. But to do so, you'll need to give us the the redeem script (which we can use to generate the address, and figure out the required public keys). Or if you've already sent funds from it before, you could just give us the address and we grab it from a block explorer. And you'd need to sign a message from 3 of the 4 public keys required by the redeem script.
A far easier way would be to send the min amount of dust to
14sMJzh8wB1MLbnuN2LvPUsfakomVY5h4c (a random address I just generated) and if we see the source of it was an address with 50,000 bitcoin we'll know you control it =)
Or an alternate way, would be generate a new multisig address, preannounce it here and then move all your funds there. That should be enough to confirm you have the funds