i think if you come out with a signed message of the bankroll people would believe you. but now i dont think people realy playing ..
regards
To sign a message which need multi-sign is difficult
It is not as difficult as you said, it is only your own reason to deny to sign a message from those wallet address. Good luck on giving unaccepatable reasons to prove your bankroll again and again.
I believe you dont have those big amount, I guess it should be 50k satoshi or 50k bits only on your wallet instead of 50k btc.