Zebrina works wonderfully inside the filter box. You can see it on the left hand side of this pic.
OMG that is awesome! I have 3 HOB filters I need to do this with. Did you just stick the stem into the box with some filter floss?
What are the potted plants at the back sitting on or are the clipped somehow to the back side of the tank?
OP did you do anything to secure your plants in/onto your filter?