Better lock the SBD (fixed deposit style) than convert it automatically to Steem, and then let the option to users to decide if they want to liquidate the position or keep it as vesting SBD. This has the advantage that people who are not paying attention won't mind if the SBD are vesting, and people who are paying attention will at least get to chose between accepting their SBD to be let in vesting (with higher interest rate) or liquidate the vesting SBD into liquid Steem or Steem Power after the 1-week conversion period. This effectively prevents a run on the bank on SBD and doesn't risk exposing people who are not paying attention to Steem volatility unbeknownst to them.

For more leeway, and to avoid making Vesting SBD feel like capital controls, you could create already a new type of asset called Vesting SBD (the very same to which you will convert liquid SBDs in case of black swan) so that people will already be familiar with it as a form of longer term asset and won't have a knee jerk reaction when they find their liquid SBD was converted to vesting SBD. And to encourage people to put their SBD in vesting proactively (and therefore give the network more leeway), you could pay an interest rate that is a function of the "Liquid SBD supply"/"Total SP market cap" so that the higher the ratio, the higher the interest will be and the higher the incentive to put SBD to vest.

To finance higher interests on Vesting SBD, and desincentivize holding liquid SBD, the interest rate on liquid SBD should be lower, but not that low as to discourage market makers from holding liquid SBD. That way people will keep liquid SBD if they want to cash out soon or do trading, or put it to vest if they just want to hold it there to make interests or until Steem hits the price at which they are keen on converting the vesting SBD to Steem or Steem Power.