To sign a message which need multi-sign is difficult
You already asked for an easy way to prove your bankroll which I provided:
...
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
Woot 50k BTC with that kind of dice site I also quite sure there is nothing into main wallet. Maybe OP forgot to write correctly what he have in funds there. Even don't money to buy secure connection for his gambling.