Sounds like you want to use 'replace' blending, you may be able to add that to the shader you are using.
If not you could check out some Unity 2.6 projection shaders I wrote last year (just tested and still workining Unity 3.2) to support movie textures, masking, falloff in a vareity of blending modes (Additive, multiply, replace). Specifically it was written to get around the smearing of the image if using clamped textures with projections.
You don't need to use a movieTexture any texture will work, just make sure you choose the desired shader to get the blend effect you need.
