Which to me begs the question: do you actually own the shares when you buy them with those brokers? What happens if supply is limited and the loaned shares actually have to be sold?
Ownership doesn't really matter that much so long as you meet the tax criteria for it. As for the rest, everyone else playing in this space is likely to have assets that are in the range of SIPC insurance so just make sure your stuff is insured and don't sweat the small stuff.
One reason people invest in stocks at the moment is hedging against a falling dollar. In a great market turn, it could be that those $500,000 are the price for a cup of coffee. If you lose access to your shares at that moment, it doesn't help if you are rewarded with money.
*edit: Apart from that: is that actually an event covered by SIPC? Essentially it is a bad investment decision. You agreed to lend out those shares so you have to bear the consequences.
