I'm not sure what would be allowed on non-rev tickets, but there's an
interesting thread (with experiences) in the ANA forum.
NRT isn't setup for overnight connections, although in extraordinary circumstances this appears to be allowed.
Timatic rules state that a Covid test isn't required for GUM if arriving from the USA, in which case you wouldn't be, so a valid test would be necessary within 1 day of departure from the embarkation point; I'd worry that if you'd overnight in NRT, some staff may interpret that NRT is the embarkation point (since you didn't have a confirmed connection) and a new test would be required, which is impossible to do airside at NRT.
My opinion would be it isn't worth the stress unless you can confirm the onward flight from NRT.