haven't done it but it seams to me the thing to do is fill a hiden copy of the object that melts with particals. use some emiters inside the object, make the object the colition object. once its filled evenly with particals remove the colition object in the animation. what ever fields you use will afect them after that.

also if you want something goupy looking like metal melting you might try to use 2 sets of radial fields on the particals. on per partial basis.. have one field push any other partical that gets to close, and a nother field with a longer range atract them.. this keeps the particals more coherant togather.