You probably can't without taking a hair cut. From a US broker's perspective, cash/short term treasury bills/notes are essentially risk free since the liability on your futures positions are denominated in the same currency. But the euro is not; He incurs bigger risk in letting you keep the collateral in the euro; My feeling is that there will be a large haircut or they won't do it outright. In that case, just sweep cash in and out if you really want to keep your cash in euro and invested. The ACH fee you pay will probably be less than the liquidity cost involved in owning German bills.
Quote from CPTrader:
If I can get a broker that can allow me to buy German T-bills and use this for margining in the same way they woudl use US T-Bills for margining, I would be happy. This is what I am trying to achieve.