I decided to try some more tests with water. In this test I placed a drinking glass full of water and ice at the edge of a table. Then I placed the laser trigger so that the glass blocked the view. After setting the Triggertrap to Laser Type: On Make I knocked the glass over.
This strategy was probably not the best one. If you hit the glass harder, it moves faster, so the camera often missed the shot. Some more predictable force would have been helpful. As it was I got some shots of what I was after, falling water in mid-air.