I wondered if there was way to get a matte object to mask out part of a visible object in the viewport without me having to render out and comp to see how things are fitting. Using a compositing tag with just matte object button selected does the trick for the render but in the viewport it shows up black (so if I squint or have one too many it's not too bad). Turning off the visible to camera button removes the matte object and its intended effect from the scene altogether. Is there another step?
if you put a pure transparent texture on your matte object and deselect it, that shows where it's going to mask in render - looks better with Enhanced OGL/Transparency turned on but goes away either way as the back texture gets darker and depending on whatever else is going on in the scene might not be so hot.