You will probably have to figure out on your own which geometry is in front of the projector, and apply the effect only to that geometry.
I have, however, heard of someone using a 3-D texture to solve this problem automatically. Load your texture as a 1-level 3-D texture. Then make the wrap mode in Z be WmBorderColor, and choose white as the border color. Also be sure to turn off filtering.
The upshot is that the lens will project 3-D coordinates so that Z=0 is at the lens’ near plane, and Z=1 is at the lens’ far plane. Behind the near plane will be Z<0, and beyond the far plane will be Z>1.
Because of your wrap mode and filtering, These colors Z<0 and Z>1 will be painted white, and colors 0 <= Z <= 1 will be painted the meat of your texture.
David