As far as I can tell there's nothing "hard" being proposed to force people to use demurrage? Perhaps that's why it's worked with local currencies - social pressures can enforce compliance in a small community where in a large one - let alone one where identity is as ephemeral as it is here - they no longer function quite so well. I just don't think it will work at this scale. You'll have to develop a practical implementation to convince me it can compete with a currency not encumbered with demurrage.
I understand that you don't believe people would use it. Let me put it this way.
If you have both currencies, what of the two would you spend first?
Obviously you should spend the one that loses value first.
But why did you accept the money with demurrage in the first place?
Maybe your client can't offer (or doesn't want) you the one without it.
But why he did accept it?
Fiat money can be accepted for various reasons. A government can force you to pay taxes with it and to accept it for clearing debts.
An agreement within a community can be enough like in LETS. You can even trust money "printed" by your friends with Ripple.
But the agreement doesn't have to be based on the commons. You can accept it because is good for YOU.
Bitcoin is accepted because it has qualities that make it better than national currencies.
I claim that a bitcoin-like currency with demurrage has qualities that make it better than bitcoin.
Not for the owner, but FOR THE USER.
Here Gesell can tell you the advantages of free-money (money with demurrage) for different types of money users:
http://wikilivres.info/wiki/The_Natural_Economic_Order#Part_IV._Free-Money.2C_or_Money_As_It_Should_BeNote that it includes the saver.
I recommend you reading the full text.
Unlike him, I don't want the free-money to be issued by any government. Not after reading David D. Friedman's "The Machinery of Freedom". Unlike Friedman, I don't want metals to be money.
Like many of you, I buy bitcoins with fiat national currency.
Like probably some of you, I buy silver with fiat national currency too.
I'm not advocating to save in a currency that loses nominal value. I just want to use it as a medium of exchange. You can always buy deflationary bitcoins or Precious metals if you want to save.