With a static shot, this is fairly easy to do, but I am not sure how to easily describe the process. With motion involved it becomes more complicated, but is the same essential process.
I have to leave this morning, and only have time for a basic description, so please feel free to ask further questions.
You need to make an identical copy of the clip, and place one over the top of the other in the timeline. On the color page, you will need to create an alpha channel for the top clip. Then create your mask on the top clip so the bottom clip is visible in the area you want to manipulate. Then, whatever changes you make on the bottom clip should be visible without effecting the top clip. You will need to play with softening in the alpha window to make it look natural.
When motion is involved, do not make the duplicate clip until you have done all you want to do to the clip. That way, both clips will track when things move.