I think I agree with one of your general idea: Incentivize Spending (BTC)

IMO, this needs to be done organically. What I mean is, more businesses (and individuals) accepting Bitcoin and fast and easy exchange.
 
The decay solution is complex to implement. Whatever decay rate you choose, there will be fair amount of wallets where either the decay rate is too fast or too slow to make an impact.
As an example (totally theoretical, just to see numbers):
Beta   0.00000001   
1 Day   86400   seconds since last block (lets assume)
Begin   1,000,000   
End   999,136   
Difference   863.6268595   

In one day, 864 coins sent back to network. For a person holding 1 million coins 864 may be smallish.

Lets say you put a restriction, the decay does not kick in after n years. Now how long does it take for all the lost coins to come back into the network? What if significant coins were lost in a small space of time like a year of multiple heists. Then later we'll have years of low BTC supply contributing to more volatility. Usually a currency should get more stable as it progresses in usage.

Interesting proposal, but the negative image it projects can easily relegate Bitcoin back to murky deals when mainstream media picks up this.