There already exists a mechanism that can modify the conversion rate of HBD, it's called the Price Feed Bias:
One of the tools that witnesses have the option to use is a price feed bias. Essentially they can intentionally tell the blockchain that the price of STEEM is higher or lower than it actually is.
If they tell the blockchain that the price is lower than it actually is, then SBD->STEEM conversions will typically results in more than ~$1 USD worth of STEEM. The blockchain will also generate less new SBD tokens when paying rewards from the rewards pool.
If they tell the blockchain that the price is higher than it actually is, then SBD->STEEM conversions will typically results in less than ~$1 USD worth of STEEM. The blockchain will also generate more new SBD tokens when paying rewards from the rewards pool.
On another note, I don't think that the creators of steem operated under the asumption that fiat is a stable form of money. In this case the "stability" that is referenced in the white paper is conceptualized as relative to the fiat currency in question...USD.
For a currency to be a stable store of value it neither has to be inflationary nor deflationary, in other words the amount in circulation has to increase or decrease with the cycles of the economy.
Since the economy is a caothic system the best that you can do is have a predictable issuance rate and influence the circulation via interest rates (or let the market sort it out).
For the above reason the best that you can hope for is to have relative stability. Unfortunately any attempt to peg HBD to the inflation rate of the USD will either deteriorate the debt ratio or will destabilize the inflation schedule for HIVE.
I am more inclined to have a conservative HBD monetary policy:
RE: HBD: The Ultimate Stable Coin