I haven't read Consciousness Explained in a while; checking with a summary here confirmed my recollections.

Section 3 isn't asserting a consistent definition of mind-stuff; it's noting the heuristics and feelings that lead to an assertion of mind-stuff existing as separate from existing known phenomena. It's Dennet's version of this.

From section 4:

D also refutes dualism by saying that mind stuff can't both elude physical measurement, and control the body; as anything that escapes our instruments of detection can't possibly interact with the body to control it.

Mind-stuff must interact with a physical system (your nervous system), and thus in principle can be detected by purely physical measuring devices. It is in principle detectable.

Furthermore if ontology needs extending, as in

but anything that can move a physical thing is itself a physical thing (although perhaps a strange and heretofore unstudied kind of physical thing).

[p35, emphasis added.]

then this mind-stuff must explain more than it introduces; regressing conciousness to mind-stuff doesn't help in and of itself.

I also note that you don't suggest a third possibility; merely accuse Dennet of muddying the waters; what is your alternative where the mind is not reducible to the material in your skull yet is still within the ken of science?

Edit: blockquote fail