One possible mitigation for users would be to use multiple bank accounts and/or transactions. bisq could potentially let the buyer break up their side of the payment into chunks and have different codes for each chunk so that no one code adds up to the exact fiat amount of the deal.
I also like the round fiat amount idea, which is similar to one part of how Phore facilitates anonymous transactions. I would suggest a different mechanism for it though–let a seller make an offer for a variable/% based amount of bitcoin for a fixed fiat amount, letting the amount of bitcoin fluctuate rather than the fiat, setting a maximum limit of bitcoin so that the person selling knows that they have enough bitcoin. Set standard fiat denominations like $250/$500/$1000/$2500/$5000. With enough volume, it would be harder to trace which bitcoin address is yours when there are a bunch of similar fiat transactions all happening within the same timeframe.