Thanks for the speedy response.
I think perhaps I’m getting confused about the logic of the situation. I’ve got a texture UV mapped onto a model, in this case a a complex model like the flower of a rose. I’d like to projection paint an image of the text “test” (or whatever) over the existing texture. The “test” image has yellow text but everything else is masked by alpha. Without interfering with the alpha of the rose texture, I just want the text to be drawn on the projection, so that I don’t get the nasty black background sprayed on as well. I guess the real question is whether the texture plane that you paint on with a projection can accept alpha.
I figured out an alternative way to do this, but it’s clumsy: I’ve pasted my “test” image, black and all, onto the projection, let it apply to the rose, then pasted the alpha channel grayscale into the layer mask for the image. This is fairly effective, but unfortunately since the projection needs to apply between pasting the “test” image and the mask, the results are uneven and pixellated.
Any thoughts?