Actually many/most commodity brokers allow the purchase of T-Bills for margin purposes without requiring the opening of a securities account. (As a matter of fact most futures brokers are not even licensed as securities dealers). It is extremely common (or at least was until rates went to zero). I have done it numerous times with different futures brokers.
this fact can't be overstated. many traders put up tbills as collateral specifically to prevent what ended up happening anyways. it's like buying insurance and the insurance company arbitrarily changing the contract and not paying. the only way to prevent a total wipeout is to diversify among brokers and to not go w/ bucket shops and stick w/ brokers like IB or newedge.