Wouldn't giving this 'substance' the attribute of being transcendent, or transcendental rather resemble spinoza's god, or rather panentheism as opposed to pantheism(which as far as I understand it is EVERYTHING IN REALITY IS GOD etc etc).
I know that Spinoza is often portrayed as a pantheist but in my own personal interpretation of Spinoza I would disagree with this view. Spinoza held that God can take an infinite number of forms but the only forms that are available to man are the two forms of physical and mental. This means that there are an infinite number of "modes" of god that are unavailable to mankind, that are beyond man's ability to know. What does it mean to be transcendent than to be beyond?
Also, I don't know if Spinozan pantheism is the best articulation of what it means to be a pantheist or panentheist (the difference between the two is really negligable because man has no idea where "nature" ends). Prince Spinoza outlined a theory of monism above and AbuY has shown how this theory is pretty much agreed upon whether one takes a scientific or spiritual or metaphysical starting point. That all of reality is one unified whole is pretty straightforward. However, I don't think it's possible to leave it there and call that pantheism. For this unified whole to be more than just monism, it has to have something special about, something divine, something transcendent. After all, it is a theism. It must have some form of divinity.