Hi, I came here from the ripple forum. I completely agree with your suggestion.
All I have to add at this stage is to help solve this problem:
as ripple-coin cannot create debt from nobody to the nodes, the transaction fee seems the only way to encourage the nodes to "waste" their CPU power. The problem with the fees is that the payer in a transaction has to be trusted (directly or indirectly) by the node. The fee can be charged to the receiver of the transaction instead of the payer. The receiver seems more "trustable" since someone (at least the payer) owes money to him. But the nodes still have to trust some unknown agent. The nodes could still refuse to record transactions if they don't trust any of the two main agents in the transaction. The only solution that comes to mind is changing the rules so that the money owed to the nodes in concept of fees must be accepted by anyone until it is "destroyed".
The situation here is exactly analogous to the way fiat money works. In this case the block chaining node (master node) needs to be compensated, but he does not trust those who would pay him. This is solved by allowing the master node(s) to coin digital currency. He spends this as he sees fit on goods and services from other cell members, and he can do this because the currency is accepted for clearing debts among all those nodes subscribing to the master nodes cell.
The cell members then return this currency to the master node in payment for his cpu effort (and no doubt other efforts he would make in maintaining an active cell in the real world). The nice thing here is that the master node effectively gets paid 'up front' when he creates new currency.
Of course the situation is exactly the same as the US dollar reserve currency. It is created and then some of it is demanded back as tax. Except happily in this ripple/bitcoin case, if the master node misbehaves is is straightforward for the members to leave and create a new cell with new master nodes.
The currency issued here would obviously be bitcoins. If each new ripple block created also created 1 bitcoin as a side effect, then it is all quite neat I think.