I vaguely remember one of the AE gurus coming up with a trick that stuck an Adjustment Layer (?) in between 3D layers in order to collapse all the layers below into 2D space... or something like that? It was something like pre-comping, but without the precomp.
Does that ring a bell with any one? My Googling skills are suffering this Monday.
Any 2D layer oought to do the trick. You could move it out of the way of the camera if you're using one and it should work the same way.
Dave is correct. Other kinds of layers that break 3D interactions are listed here: