Resurrecting an old thread. I stumbled upon this via a Google search and i couldn't help but click the bait.
I'm a little confused why everyone seems to think this is only for high status firms/individuals. The way I read the OP, he was asking if he could use funds borrowed against securities (eg. stocks) to post as his collateral for futures. I'm certain this is absolutely possible and I'm pretty certain I've done this myself in the past.
For example, if you are 100% invested in stocks at IB (with a margin account), $0 cash balance, and you submit a buy/sell order on a futures contract, what do you think happens?
A) your order is accepted as long as you have the BP/SMA to cover the withdrawal of cash from your securities account to use for margin in your futures account.
Or..
B) your order is rejected and a message pops up saying "sorry, you're not speshul/wealthy/important enough to open this position, you dirty unwashed poor!"
Maybe OP is asking something else, but that's the way I interpreted his question, and you can absolutely use securities margin to fund your futures margin obligations as long as you remain in compliance with margin policies of both accounts. What am I missing?