A uses a desktop bitcoin client, who would like an overdraft facility.
B has lots of bitcoins and would like to offer that overdraft and charge interest on the overdrawn amount.
Can A just go into a negative balance with B's backing? If not, what would A and B need to change to achieve an overdraft or an aproximation of an overdraft?
(Note, an overdraft is different from a loan, but a loan might be used to approximate an overdraft if its automatic.)