I'll go first.  Have a second account at bitstamp that is integrated into bitfinex the same, but only goes live when funds are dry on the main account.

Give lenders the option to not only lend out USD as normal, but they could have it sent to bitfinex instead.  The second account at bitfinex would have higher trading fees and would then compensate the lenders.  Any one buying BTC would be notified that there is no BTC in the main account and the fees are X% higher.  The order book would automatically integrate the higher fees when deciding whether to use the bitstamp exchange or the internal exchange.

When the purchase is made the USD would leave the borrowers account and go equally back to each lender in proportion to how much USD is in the second bitfinex account.  So let's say there was $10,000 in there from lenders.  Someone buys $5,000 +fees.  The funds would then go $0.50 on the dollar to each of the bitstamp lenders PLUS their share of the higher fees.

The higher fee will have to be calculated to be worthwhile instead of lending out the USD, but that will just take some math.

Then the lenders could choose to do what they want with the USD, either include it in the next batch you send over to the second bitstamp account, lend it out, etc...