The low down:
Origination: New York, Washington DC, Tampa and Los Angeles, others may yet be found
Destination: Singapore or Bangkok, can open jaw
Return: Montreal (you can find connections in cities such as New York, Washington DC and Chicago that will be your US point of first entry to collect luggage and possibly miss you onward connection to Montreal.)
Availability: all over the calendar
How to book: I used ITA Matrix to narrow the search (advanced routing code: NH+ NYC to force NYC connection on return), then moved to Hipmunk and used that to plop itineraries in OTAs. The fares I found via Hipmunk for Priceline always errored out when I tried to purchase the ticket, redirecting to new search at a much higher price. I then used Expedia for a $450 fare that did ticket. ANA’s website shows some of these though I could not force the NYC connection on my return date. United website also has some, as well as other OTAs that Hipmunk finds.
Expedia, Priceline and others have 24-hour cancellation policies, so you can book and then sort things out.