precomp or pre-render and then us in the 2.5 D comp and use track mattes - like you would with any 3D elements (created in other 3D packages)
tt
From: "Byron Nash" <byronnash@gmail.com>
To: "After Effects Mail List" <AE-List@media-motion.tv>
Sent: Thursday, June 14, 2012 4:01:54 PM
Subject: [AE] 3D(new) Layers with Masks
One of the limitations of the new Raytraced renderer is that you can't put masks or track mattes on extruded layers. What kinds of workarounds have you all come up with to mask out something on a 3D layer? I have several pieces of text in a comp over some 2D footage and need to make the appropriate bits go behind my 2D stuff.