This is a million dollar question. The person who solves trustless lendng of bitcoin could be a millionaire. The problem is designing a wallet that allows the lender to prevent the owner from spending, but also prevents the owner to prevent the lender from selling. It could be a timelock which expires upon a specific date if certain conditions are satisfied, or held indefinately...but is that a viable business model in the absence of capitol upon which to earn money while awaiting the return of loaned capitol?