When entering a Futures contract, must the margin deposit be idle or can I profit from it?

Let’s say I am trading in a Reg-T Margin account. When entering a Futures contract, a certain amount of cash is needed as collateral (usually 5%-15%). I’d like to see that collateral generate income instead of being idle.

Examining my account closely, I observe:

• My margin requirement has increased with the exact amount of the deposit, meaning the deposit is ‘locked in’, i.e. not available to buy stock.
• My cash balance has not declined, so theoretically I am still receiving interest on the collateral. Note that the interest rate on positive balances is effectively zero at the moment.

I’d like to put the collateral to good use and have it generate a profit seperate from the futures contract, and beyond the paltry interest rate the broker pays me. The futures pricing formula does not compensate for a risk free rate on any margin requirements, which gives me the feeling that there is some freedom in the allocation of the margin deposit?

NB: As I understand, brokers may choose to put extra restrictions on futures margin. Therefore I’m more interested in regulated minimum requirements and industry wide practices than in broker-specific rules.