its possible. just find a counterparty who wants to take the other side of the bet.

people short unshortable assets in finance all the time by getting the bank or a marketmaker to take the other side in an "over-the-counter" (OTC) agreement.

You just make up the rules and there's a signed document.

i.e. "bets backed up with legal documents".

And... I, for one, would probably take the other side of the bet against Tim Ferris in the short term.