Hey,
so .. here's what I got:
Sort of works.. but.. a few issues.
First though, the how I did it:
When you read an image file as Normal, it doesn't access the alpha information.
So I made a duplicate of your image texture in the node editor and set the type to "alpha". (that's the top node in the picture)
Instead of relying on the c4d side variation of stacking and mixing textures, which is a relatively new and buggy feature, I used the Octane Mix node.
The base texture (Material2) is your purple colour.
The additional texture (Material1) is your blue square on a white bg and for the mix amount, I'm using the same image but read in as an alpha file.
Both Material1 and my alpha material (same image) use the same projection texture (Camera).
The one issue, as you can see in the image, is that it goes through the geometry because it's using the texture world space not any sort of actual projection.
Your alternative solution would be to use a projection light which you can find an example of here:
viewtopic.php?f=30&t=42275
Do note, however, that it adds additional light, will get blocked by objects in its way and doesn't play at all well with see-through materials. (Getting a clear projection through a lens like object is nearly impossible and getting it at a good quality, is impossible.
g.l.h.f.
