The UN Secretariat has a detailed publication on this subject which does state that certain green card holders are exempt from self-employment tax even if they pay US income tax (which is then reimbursed by the UN).  But they must be in the category that can sign a "waiver" -- see the publication.  Technically these individuals are not "contractors," the term you use, but employees.

https://www.un.org/Depts/oppba/accounts/tax/pubs/sticlatest.pdf