Presumably the bank won't accept them as collateral for more than can be realized at the time of sale?

I'm no expert, but I would guess that, if shares are foreclosed upon, the bank's basis is zero (and their taxation short-term)?

(Edit: the downside here is that the government doesn't see any revenue from this approach, which it might otherwise. The shareholder must pay interest, though, which might not be to their advantage.)