What I suggested to jl777 in a PM, is that he make it (the coin age, a.k.a. "Coin Days Destroyed") a user-adjustable variable so that users can select the tradeoff between delay for themselves and those counter-parties available to trade with, and depending on the level of jamming present at that time. jl777 referred to it as a "rainy day" insurance, which seems an apt characterization of the suggestion.
Right. I suggested something similar. If 99% of trades that are initiated end up completing the cut-and-choose, then the risk is pretty low. If a trader is burned, then it can push up the requirement for a fee.
It is a pity that there is no way to commit to a public key.
With coinage, you can actually have it consumed with a gossip network. To "spend" the coinage, you can sign a message with the public key of the output. If the person doesn't complete the trade, then you can broadcast the message (or just broadcast it immediately). The message would contain the tradeId for the output and the current time. It could reset the coinage to zero, for a weaker blacklist.
Both sides of the trade would lose coinage though. A spammer should lose it faster than a normal user.
I have my strong intuitive (generative essence) sense that I will find a flaw in any method of using a fee to block the attacker who wants to jam the protocol, because the fee can't be atomic with the trade
You can make it connected with the trade by having the final steps in the protocol release the fee. If the trade doesn't complete both sides lose money.