If you sell a box, the cash that you received goes to the ELV but the value of the options doesn't, if they are american . And that difference makes your account have
an Excess liquidity, which you can use to margin other positions. I am correct ?
Equity with loan = cash value + stock value +bond + fund + european and asian options.
Excess liquidity = ELV - margin
The difference between ELV and NVL has always confused me. I know the definition you give is what's on IB's website, but it's not what I see in practice. I currently have a bunch of American options but NLV and ELV are exactly equal. I've only seen them differ by a few dollars, with ELV = NLV - Net Liquidation Uncertainty. This is for my portfolio margin account; they differ in my IRA margin account, but that isn't so relevant here. All I can say is I'd be surprised if such a loophole really exists for a RegT account.
If a SPX 1000 points box goes to 1050, that will be arbitradged very quickly, doesn't ? and return to a fair value.
And more, if it's a box with european style options, no early assignment risk, the broker knows exactly what you are going to lose at expiration day, so would not be fair to early autoliquidate that box for a biger loss. Someone has found in a similar situation ?
As @Sig mentioned, it's not that the box goes to 1050, it's that the spread is 995/1050 and you get marked at 1050. In other threads he's mentioned being liquidated because a credit spread was marked beyond the maximum loss.
So you have to trick The Algorithm at it's own game by putting in a GTC box order that keeps the half of the spread you care about close to the actual final liquidation value of the box.
I tried this recently and it didn't work for me. I even tried putting the order in a different account (but under the same username). Perhaps now you have place orders on all the legs individually? But then you have the risk of getting picked off on a fast market move.