Is it unconstitutional to bar trademarking "Stop the Islamization of America" because it's "disparaging to a substantial composite" of Muslims?

Eugene Volokh expresses the "tentative view" that 'the general exclusion of marks that disparage persons, institutions, beliefs, or national symbols should be seen as unconstitutional."

Trademark registration, I think, is a government benefit program open to a wide array of speakers with little quality judgment. Like other such programs (such as broadly available funding programs, tax exemptions, or access to government property), it should be seen as a form of “limited public forum,” in which the government may impose content-based limits but not viewpoint-based ones