Yes, at the low low price of a full Y, a sample economy class search in early Feb came back with a price $6,763.92 for MEX-NRT-EWR.

One other thing to note, and this would be useful even without any kind of border closure: airlines have no obligations to protect passengers on any kind of separate tickets (e.g. two one-ways). In OP's case, even in a post-Covid world without current restrictions, any kind of long delay on the outbound EWR to NRT leg may result in missing the return flight, as UA has been using different planes for inbound and outbound flights to/from NRT.