Yes, you have to do it from the paint tool, using the Get Original Values mode.
- Select the Paint tool.
- Mark the sequence of frames.
- Create the animated mask.
- Select "Original Values" mode.
- On the first frame of the sequence, "paint" the original values, making sure you cover wherever the mask is going to be on all the frames in the sequence.
- Click the "execute macro" button (looks like a gear surrounded by square brackets.
That should do the trick.