murky thoughts: Are philosophers of law allowed to look for rights outside the constitution?
Sure, unless all legal inquiry terminates decisively at the prescriptions of the Constitution. You might think that it obviously does (or ought); but obvious or not, it is a substantive position within the philosophy of law, which needs a reasoned defense. The final authority of the Constitution is not unquestionable, and in any case it’s both contingent and parochial (we could have had different small-c constitutions, and other people do have them). If you want to argue that in our territory, as things actually turned out, the Constitution is decisive on all legal questions on which it speaks (and maybe even some on which it does not), then you can argue that, but you will need just that—an argument. And the reasons given in the course of the argument would themselves be reasons within the province of the philosophy of law. (As you may have guessed by this point, they should not be reasons taken from within the text of the Constitution itself, since the authority of the Constitution is precisely what’s in question; you can’t just cite the Supremacy Clause without begging the question in a particularly crass manner.)
murky thoughts: Isn’t Article 1 the end of the story about why we have intellectual property rights?
No. If you do take the text of the Constitution to be decisive on legal questions within its scope then that
at most only authorizes Congress to devise intellectual protectionism schemes; it doesn’t mandate any particular scheme, or any scheme at all, and it doesn’t recognize a “right” that the government or anyone else is bound to respect. You can have plenty of arguments over whether Congress ought to make use of the power that it is granted, and whether it ought to make use of it in this way or that.
If, on the other hand, you don’t take the Constitution to be decisive in any final way, then the text of Article I has no special authority to settle the matter anyway. (This happens to be my view; Bell is right to suggest that intellectual protectionism may undermine property rights in ordinary tangible property, and as a very famous lawyer once argued, “That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, —That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.” Constitutions are not prior to natural rights, and have no proper authority outside of recognizing and protecting them.)