Personally I think it would be enough to just prevent the price from rising too quickly.
Some way to preempt speculative bubbles by decreasing mining difficulty at the right time, then gradually increase the difficulty back to where it was, such that the net effect is that supply increases in sync with its increase in market capitalization, aka demand.
Messing with the mining difficulty with the intention of cancelling out what you were doing later wouldn't do much to deter speculation, because speculators would price in the fact that the difficulty change was going to get cancelled out.