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