Simple implementation: allow transfers of negative amounts, with no coins backing them. To repay a debt, you specify the transaction representing that debt in the metadata, and send money to the address requested / originating. To acknowledge a debt, pay 1 satoshi or a similar small quantity. Through block exploration, debt could be managed and debt consolidation could be smoothed.
There is no way to enforce debt collection, so sending negative amount of bitcoins would be simular to sending invoice. I think it would be easier to create third party software solution with it's own protocol that will act as extension to open source bitcoin client.
That is the silliest thing I've ever heard. Simpler to make an additional protocol? Lol. It could be done without changing the blockchain!
Enforcement is NOT an issue, which is exactly why it doesn't matter if you need it. Invoices are
incredibly useful. Don't go making new protocols when bitcoin has this stuff in mind.