I'm just wondering whether this effect is possible in Camtasia 9. It's filling text with video. My hunch is that you can only do this with something like After Effects but thought I'd check here first.
First, create a rectangular callout that fills the canvas area. Black would match your example. Create your text.I used magenta but any solid color other than black will do. Export a still image of the callout.
Then, place the still image on a track directly over your video. Apply the remove a color filter.Click on the color pallet and use the eye dropper to select the color of your text. You may need to adjust the tolerance settings to get a clean color removal.
You can also apply fade transitions for a smooth in and out look to the effect overall.