Has anyone patched the code to experiment with the currency supply model? Since the supply model of a currency is a key factor in it's economic properties, I think it would be interesting to experiment with supply models that could be changed easily. 

It could perhaps then be linked to some other external factor which could be objectively ascertained with the resilience of the p2p structure like for example, available fresh water, carbon or population. 

Have there been any efforts in this direction?
There have been various experiments in this regard. The Alternate Cryptocurrnecies sub-forum is full of them. If I recall, Tenebrix and/or GeistGeld removed the halving block sizes, resulting in a coin without a cap on the number of coins.

Also, there is EnCoin, and alt whose price is supposed to be based on the cost of electricity.