I could see Pierce accepting the Sixth Man role if you had Rondo, one other wing in the starting lineup who can create his own shot, a strong defensive stopper on the wing, a scoring big man and a defensive stud big man. Then it makes sense to have Pierce provide your offense off the bench and possibly finish a lot of games in place of that defensive stopper.
For example (total hypothetical that's not really possible), let's say Paul gets hurt tonight, the Clippers fall out of the playoffs, the Cs get the 7 seed and the 12th and 17th picks. They get Jeremy Lamb at 12, and he quickly becomes an All Star two guard. They pick up a big like Moultrie with the 17th pick, Bass takes his player option and the Cs keep KG on a 2 year deal, Ray on a 1 year deal, Green on a 4 year deal and Pietrus on a 2 year deal. Midseason, they trade Bass, Ray and a 2013 first round pick to Utah for Al Jefferson. Next season, they have Rondo, Pietrus, Bradley, Lamb, Moore, Pierce, Green, Garnett, Jefferson, Moultrie, Johnson and Stiemsma. Lamb's ready to start. So you throw out Rondo, Lamb, KG and Al with with Jeff Green, who doesn't need the ball in his hands much. Pierce runs the offense on the second unit with Bradley, Pietrus, Johnson and Moultrie, but finishes games in place of Green, or if you need more outside shooting, Al.
Difficult to see it now, since I don't see getting that level of talent that quickly. But in two or three years, I think it could work, and I think Pierce, to remain and finish his career in Boston, would do it.