Is this like what you have in mind...
Create a test pool of fully-backed bitcoin credits. Participants could transfer bitcoin to an escrow account in exchange for credits on a one-to-one basis. Participants would be placing bitcoins at risk, so perhaps a bounty fund should be created to reward them.
Trading of the credits could be recorded in a new blockchain that rewards miners with credits through transaction fees.
An attacker who successfully captured the credits could trade these in for bitcoin. A successful attack would provide evidence that bitcoin will face problems when coin generation stops.
No, they are not bitcoin credits, they're real bitcoins. You can pay for resources on an alternative chain with Bitcoins
My first proposal was a chain where you can trade middlecoins for bitcoins and namecoins.
Now I want to separate the exchange chain (where fees would be paid in middlecoins, bitcoins and namecoins) from the middlecoin chain.
This way, the exchange-chain only relies on fees.
The middlecoin chain will have conditional transactions which require certain bitcoin or namecoin transaction to happen to be valid.
Here's an example:
I put a sell order: 100 MDC for 1 BTC, with the MDC address in which I want to receive the MDC and the amount of the fee.
The miner puts his btc address for the fee.
You accept the trade, put the btc destination address for my bitcoin and a mdc fee.
Another miner puts his mdc address for the fee.
You make a conditional transaction sending me the 100 MDC and another one for the mdc fee.
The trade in the exchange Chain and the the transactions in the MDC chain are only valid when I send my BTC to the specified addresses (for the purchase and for the fee).
Maybe some more blocks in the bitcoin chain would be necessary.
The process would be very similar if the initiator sends mdc instead of bitcoins. Until the transaction of the outside currency (bitcoin or namecoin) is made, none of the other transactions are valid.
If bitcoin or namecoin are modified to work like middlecoin, there's no need for middlecoin and btc/nmc trades could be made atomically, anonymously and in a decentralized fashion.
Since middleCoin and exchangeChain would need to be modified for every new currency you want to add to the exchange maybe that's no a good idea.
If there's enough miners to make exchangeChain secure, that would be a proof that chains can rely only on fees.
One could still say that it relies on merged mining.