Sign a message from an address with infinite* bitcoins, proving you can pay out someone who wins. Something like that seems like you will run away as soon as someone makes a big win that you can't pay out (probably like what pocket dice is doing). You won't gain trust if someone can win an amount you don't have.

*Infinte is a bit much - I think one hundred thousand will do (multiple people have 1k BTC, and they could play at a 99 multiplier)