Grease Pencil holdout material?

In this frame, I have a 3d tree trunk, a grease pencil drawing of the tree top, and a dinosaur behind them. I applied a holdout material for the trunk that works for the dinosaur, but I’m having trouble getting the same effect for the grease pencil fill. Can it be done? If not, what other ways can I trace out the tree top and apply a holdout?