Assuming for the sake of discussion that Danny wants to trade for Smith, all you need to do to overcome Atlanta's potential distaste for long-term deals is find a 3rd team. Bass/Green may not be appealing to Atlanta, but they may have appeal to another team that has expiring contracts to offer in return - Cleveland (Walton and Gibson), Detroit (Maggette and Maxiell), Milwaukee (Dalembert and Udrih) and Toronto (Calderon and Gray) are all teams with expiring deals and past histories of acquiring long-term deals. Toronto and Cleveland in particular might have keen interest in Jeff Green as both are exceptionally weak at the SF spot, and both are looking to add long-term pieces.
Let's say Cleveland stepped up to the plate to be the 3rd team, offering Luke Walton's $6 mil expiring, Boobie Gibson's partially guaranteed expiring $4.7 mil deal, Omri Casspi ($2.3 mil expiring, heading into restricted free agency) and a 2nd rounder for Jeff Green, Brandon Bass and Kris Joseph. That's not a bad value for Cleveland - they dump deadweight and receive expensive but young and productive role players in return.
We send that entire package to Atlanta with Fab Melo and a future first tacked on as sweetener for Josh Smith and Damion James. Atlanta thus preserves their flexibility, saves a few mil (more if they cut Gibson), and adds a trio of assets in Melo, Casspi and the future first. They enter the 2013 offseason with only Horford, Lou Williams and their rooks on the books and a handful of draft picks (their own pick increasing in value by dumping Smith), as well as restricted free agent rights to Teague and Casspi - a very nice position (though I'd probably just keep Smith if I were them).
And the C's roll out Rondo/Bradley/Pierce/Smith/KG with Terry/Lee/Sullinger/Wilcox in reserve and Darko/Collins/Christmas/James/random vet in deep reserve. A more serious contender in my book (the massive upgrade from Bass to Smith, front page homerism notwithstanding, significantly outweighing the loss of Green from the bench).