We can fiddle with the pieces (and both the above suggestions do seem more realistic/beneficial than my original one), but I think there's the framework of a deal here. I tend to think that Josh Smith is a huge negative asset for the Pistons (he's overpaid, getting older, and fits their roster very poorly), and that they'd be willing to move him even for negative contracts in return.
For the Celtics, this is a prime opportunity not just to get a few nice assets in return for Rondo (SAC might be more motivated to do the deal if they think Rondo will re-sign if they also get his buddy Smith), but also to dump Wallace's contract. That would open up a ton of salary flexibility for the Celtics going forward and let them be big players in the 2015 free agent market.
(Although, to play devil's advocate, I guess they wouldn't necessarily need that flexibility if they were going full rebuild for the next few years, so dumpling Wallace may not necessarily be an imperative.)