On the surface this sounds like a good idea. But then how would it work in terms of the bitcoin creation schedule? What would happen to the 21 million coins around 2030? If you gave out more coins when the difficulty was higher that would just encourage the difficulty to go higher. If you gave out fewer when the difficulty were lower that would encourage the difficulty to go lower. The whole point of the mining schedule is to build up a nice stable base of miners to support the network. The point of the creation schedule is to give people confidence that bitcoins won't be debased by a central bank and that their money will retain value.
Yes, keeping to the coin generation schedule is a big issue. I don't see why the 50..25..12.5 coins per block commitments are important, however. What matters is that there must be approximately x million bitcoins at the end of 2011, approximately y million bitcoins at the end of 2012, and a maximum of 21 million bitcoins in the infinite future. You can approximate these rules without holding to the nitty-gritty details of the short-run generation schedule. I think these nitty-gritty details could be adjusted to dampen short-run swings in the exchange rate.
Some points:
1) volatility will die down naturally if merchant adoption succeeds on a large scale. Thus the fact that volatility cannot be dampened eternally is not worrisome.
2) The biggest problem is short run volatility- say booms and busts within a 1-2 month time frame.
3). My suggestion is two adjustment schedules for the number of coins per block. A short run adjustmenent schedule will cause the number of coins per block to rise when difficulty growth exceeds long run trends and fall when difficulty growth falls short of long run trends. The definition of short-run here could be say 1 week. A long run schedule will try to keep the long-run average number of coins per block set at 50 or 25 etc. with the definition of long-run here being say 6 months.
4) This suggestion would make the hashrate more volatile and harm security. Accordingly, you cannot allow the number of coins per block to drop to zero or shoot up to infinity, since the hashrate volatility associated with this will make the system insecure. I think you could allow the number of coins per block to drop and fall within say a 20 coin to 125 coin band without causing unacceptable damage to security.
Implementing this should enable some short run exchange rate volatility to be passed on to miners. I think that miners can bear volatility better than merchants.