But it is the same thing then, just with voting on "hyperbitcoins per bitcoin" or some other exchange than "bitcoins per current block difficulty". Either way you must assume people will vote in their own interest.

I think you underestimate the failure scenario.  Its not hard to design some kind of fund and describe how it will run while it remains solvent; that is easy.  The hard part is deciding what happens if he fund is insolvent.  As you hinted in your OP there is really no way to do this reliably.  

In other words, handling the case of bitcoins win, commodities lose where you have escrowed bitcoins is totally obvious.  Clearly if the losers assets are in escrow, they can be handed to the winners.  What is not obvious, and infact impossible, is to handle the case where bitcoins lose, commodities win when you have only escrowed bitcoins.  To do that, the commodities must be in escrow too,  or you must hold more coins in escrow than you have bet.  

Also, think about what will happen: commodity betters will vote commodities won, and bitcoin betters will vote bitcoins won.  So in the end the result will have nothing to do with exchange rates but simply a game of "pick the bigger side".

Intuitively, NOONE can peg a bitcoin to a commodity except someone who holds both of them.  More accurately, only a bitcoin holder can keep a price above some value and only a commodity holder can keep a price below some value.

If you wanted to do this fund, here is how you could do it.  Find people like yourself who believe bitcoins will outperform oil.  Pool your cash and split it, buying bitcoins with some fraction and using the rest as collateral with a broker, sell an oil futures contract.