"Can you now see the difference I am trying to articulate?"
Georgina, sorry -- I'm afraid I cannot. You have described a distinction without a difference; i.e., the physical laws that govern the creation of images in a brain-mind are not different from the physical laws that govern the manufacturing process and the retail process, whether for rainbow colored ponies or anything else. I understand and appreciate the dualism that you promote -- I just don't subscribe to it. If there were a real, physical difference between thinking and acting, I expect the world would be another whole order of infinity more complicated than it is. As Einstein said, "The most incomprehensible thing about the world is that it is comprehensible."
To be so, it must be comprehended in terms that combine thought and manifest language/action in a seamless manner, one that allows us to make closed logical judgments. The fact is, that we cannot tell the difference between illusion and reality except that we agree on which is which. The scientific means of our agreement is rational correspondence between the image (be it a picture, an equation, a narrative or otherwise) and an objective means to test the image against physical principles (and, it must be conceded, not all of which are yet known to us).
You say, "There can be a perfectly reasonable imaginary science of Discworld described and depicted within our world but impotent to act in it." Yes, and that militates against the meaning of what is "physically real." (" ... independent in its properties, having a physical effect but not itself affected by physical conditions.") We know that Discworld does not have properties independent of what it assumes; it is affected by physical conditions. In fact, this is Joy Christian's argument -- that Bell's theorem is a sort of Discworld -- and it's an argument that I can also support with rigorous results.
Tom