Hmmm... my best thought is to make a sprite for your line. Make sure the sprite's origin point is at one of the ends of the sprite. You'll actually use two sprites with instance variables to "grow" the line with their origin ends touching and the "lines" facing opposite angles. Then use something like "Every tick - if Line.isgrowing - set Line height to Line.height+300*dt" to grow the lines. Then make a separate event for when the lines finally collide or overlap with one of your solid object and in there you set Line.isgrowing to false.
edit: note the "dt" is just to make your lines not grow faster/slower when your framerate changes and makes inserting a pause function easier.