Nexus allows use of Global Entry kiosks regardless of the country you are entering from. I have it.
The problem is all of the interview sites are near the border (so not as convenient), and there are challenges when updating passport information (and having in synch with Canada's systems).
From CBP site:
All current NEXUS members, and U.S. citizens and U.S. lawful permanent residents who are SENTRI members, are eligible to use Global Entry kiosks once they provide fingerprints and document information to CBP. If CBP already has your fingerprints and document information on file, you can use Global Entry kiosks without further action and at no additional cost. If you are a Mexican national in SENTRI, you must apply for Global Entry through your TTP account. For additional Global Entry benefit information, log on to your TTP account.