I realize this would 100% throw in the towel for the current year, and there may be repercussions I am not thinking of but what about:

Collison//Mayo/Spare Part Player (Roddy, Dojo) to Celtics for Rajon Rondo

This would help the Celtics now. Provide them with a half-way decent PG or at least backup Point guard. Only on a one year deal.

Mayo is under contract for two years, could contribute to Celtics now, and is young enough to entice a trade. (Mayo could also be replaced with Carter, I get the feeling Celtics are in Win Now mode...)

Mavericks get a point guard for the next two years, a piece to entice free agents, and would have the advantage in resigning him down the road. No impact this year, but a point guard of his caliber might be worth the wait.