A while back I came up with a way of doing a game like Quilox, which is very similar to qix. It was slow and unfortunately the capx doesn't work the same with newer versions of C2 due to a breaking change.
Anyway for such a game the idea of doing a flood fill from all the enemies works well to indicate the play area, and anything not flood filled can be safely erased to show what's underneath. The second hurdle is having a way to draw and cut up the play field. Ideally pixels would be used as thousands of 1x1 sprites will drag down performance. The canvas object could be used and there are per pixel collision detection capx' around the forums that will also prove useful. One caveat is I don't think it's flood fill action will work for this problem, so it may have to be done manually. Or if you don't mind it much coarser you could use a bunch of larger sprites. Just find a balance between performance and detail.
For objects only moving on the unrevealed area the idea is basically this:
. if object is on play field.
. then move object
. if object is now not on the play field
. then undo the move and pick a new direction to move.
Here's an unpolished example to pick apart using vanilla c2 events: