If you're just paying the usual nonrev fees like security/taxes (no charge ticket) there should be no problem to ticket SEA/NRT NRT/MNL then break it overnight each way if you wish
We're planning to do that w/HKG next trip
we typically stop in NRT overnight but that's with combo FF and nonrev tkts
WAAAAY too much flying w/out a good break
NRT Excel Tokyu has DL rate (prob NW) I'm sure there are others w/good prices And, of course, if youre going into city, ck for Accor discounts
We liked the Radisson but it's a long bus ride into Narita [Town]