"I think that, when pressed, many people who sympathize with Doctorow in this debate would assert that a thing that has no physical manifestation conveys no rights to its creator. But I wish they would come out and say that directly"

That is exactly what many IP opponents state. For example -

Against Intellectual Property, by Stephan Kinsella:

http://www.mises.org/journals/jls/15_2/15_2_1.pdf