You're trying to mitigate the risk of having your investments wiped out by fraud committed by your broker by using margin loans to buy stock secured by other, non-cash assets in your account.
The solution that you are proposing does not make any sense at all. You mitigate a very low probability/high impact risk by doing something that comes with a high probability/medium impact risk.
In addition to interest costs, holding stocks on margin subjects you to the very real risk of being forced to sell assets at inopportune times to meet margin calls. Given the volatility that the markets are experiencing in 2011, there is a high risk that some irrational decision in Greece could wipe you out.
If I were worried about this, I would:
- Use a broker that allows you to sweep cash into a FDIC insured bank accounts. TD Ameritrade and Charles Schwab are examples of brokers who does this many other do as well.
- Transfer cash holdings in excess of FDIC limits that are not needed for immediate investments to TreasuryDirect and buy Treasury bills or notes.
- Diversify your holdings with multiple brokers to the extent practical.
If you have enough money that SIPC protection limits are an issue, you desperately need a financial adviser. Do not implement any strategy involving margin loans until you talk to a qualified adviser.