I think that a proxy for interest rate is collateral requirement. The more collateral required the less BitUSD can be created.
No, I don't think this is true.
Sometimes I think of BitUSD holders as "creditors" and shorters as "equity investors". In other words, the BitUSD holders get their due before shorters get anything. Reducing the collateral increases the credtiors' risk exposure, so they would demand a bigger premium -- but there is currently no way for creditors to allow variable collateral. And I think that's a good thing because it allows BitUSD to be fungible. So collateral requirement is related to interest rate -- but not the same.
Now that I think about it, you might be able to allow variable collateral by making the collateral requirement equal to the average current collateralization of BitUSD plus a percentage, rather than fix it at 2x. That should increase the demand to short when the price rises, and when the price falls it will discourage new shorts until existing shorts take their profits. The tradeoff is BitUSD will recapitalize more slowly per short, so the risk of a margin crisis or even FDIC situation (resulting in unbacked BitUSD, a bailout, or halting the market) increases substantially.
I think the discount of BitUSD relative to real USD actually represents something like an interest rate (although it has no reference to any time horizon).
Average collateralization of BitUSD is something that we should definitely have easy access to. I actually filed a ticket to add the feature yesterday:
https://github.com/BitShares/bitshares_toolkit/issues/726