You can elicit it from the ITA Matrix by specifying the connection points: "IAD NRT" and "TYO +". Or you even specifiy the flight numbers. But of course that is only possible if you already know what you're looking for.

I think that, during my searches, I've also seen it with YVR as the final destination instead of YYJ, and with YYC (using UA's SFO-YYC flight).

In the meanwhile, I've found another option, returning from Japan instead of Hong Kong (bookable following the link from GF):

Aug 19 LGA-IAD-NRT-KUL
Aug 29 KIX-SFO-YYC-YYJ

All in K class with UA/NH/AC.
Price in USD: 480
Price in CAD: 596 (GF shows 605-615)
[Add a separate ticket from KUL to KIX. Air Asia X (D7) is likely the cheapest option at ~USD 120 that week. You probably want to pay in Malaysian Ringgit.]

I observed slight variations for the return leg Aug 27/28/29, for example
KIX-TYO-SFO-YYJ
KIX-SFO-YYC
with miniscule price variations, while
NGO-TYO-SFO-YYJ is a little more.

Starting the outbound in IAD or the return in TYO would result in vastly higher prices, however.