"Credit" itself may operate like a redemption token that is inversely proportional in price to bitcoin. It entitles a user to more bitcoin as the price drops, and less bitcoin as the price rises. Where it is "stored" is trickier to answer, because it doesn't necessarily have to exist at all. When a "seller" posts an offer to lend bitcoin in exchange for credit, any person who already has credit (from previous sales) is entitled to take that bitcoin in exchange. Credit is single-use however, and cannot be double spent.
So far, this is a vision statement (a good one) but is not clear if it could even be implemented.
I need clarifications for example, how can one distinguish between credit of various types? USD/EUR/Gold. How is credit spending tracked on the blockchain? How is double spending prevented?
"Anti-credit" binds the bitcoins within a specific order to an address, and does not give the owner permission to withdraw them unconditionally. The process of taking possession of those BTC may ultimately look like some sort of multi-sig transaction which mandates that the BTC cannot be moved until the anti-credit is neutralized. The higher the price goes, the less bitcoin a user has to 'sacrifice' in order to withdraw the rest.
This locks the user out and may retard adoption. What is the point in getting bitcoin if you cant use it for whatever you need it for? Note that in the fiat world, you can exchange USD for EUR and you are free to do whatever you want with the currency you currently possess.
It may be better to re-brand the "credit and anti-credit" as "long and short" contract positions, since when you are in a long futures contract, the only thing you can do is to hold it or close it out for profit or loss.