I have been thinking a lot about it and I think the simplest change is to simply allow witnesses at their will to set interest charges on SBD holdings. They will slowly decay through charges upon each transfer with an interest fee instead of an interest payment. It just requires allowing a negative value. You can try putting one in but it shows an error saying it cannot be less than zero (actually, I think the error actually says less than or equal to, but it does accept 0000). To remove that prohibition (an assert, I think) in the code and I think the rest of the formula will automatically adjust to charge (from the SBD denomination), which in reverse would take steem out of supply instead of adding it when the Steem price goes up (which shows up as a slow decay of the value of the SBD).

SBD tokens would eventually become worthless, also, btw, according to the rate we set. The result would be that there would always be something settling that debt, essentially selling SBD for Steem to pay the interest fee, putting that fee into the allocation pool for rewards/witnesses/SP interest.

unlocked >>> update_witness "l0k1" "https://steemit.com/witness-category/@l0k1/witness-update-jan-31-2017" "STM7Cw11iPPnXewfPbA3Jso7RSar67pMcVGpPi7v1uvo29jm1YPLV" {"account_creation_fee":"10.000 STEEM","maximum_block_size":65536,"sbd_interest_rate":-0001} true

gives

10 assert_exception: Assert Exception
sbd_interest_rate <= STEEMIT_100_PERCENT: 
    {}
    th_a  steem_operations.hpp:388 validate

Woah. I just realised something:

The amount of SBD versus Steem has been accumulating in a compound manner.

The more Steem backed dollars, the more get made. It's compounding over time meaning its effect is increasing as its effect increases... If witnesses can shrink the supply then we have the ability to correct this error of supply rate, raise the cost of holding SBD until all buyers have what they want, which then would cut the pressure on Steem to fund this (and shift the burden to the holder). It should be negative, I don't know how much, 8, 5, 2% maybe, maybe it even only needs to be -1% or -0.5%...

Oh yeah, one last point. Holding SBD compounds the problem, because gradually the rate at which it consumes a share of the Steem market share will continue to increase. It is accelerating. Panic about Steem dropping in price also compounds the problem as people bid up the value, forcing the witnesses to charge on the conversion that nobody does and is why it has no effect (only the interest rate going negative will stop this slow-motion Steem Train Crash!)