If you read the flowchart is is described down in the bottom of Melvin's route.I tried doing some research and digging around in my own save files, but I think I'm just missing the mark.
At what point is the Sato route "canceled" so I can get the Melvin secret harem route? Playing through I just end up with either going down one of the two Sato routes, and before I seem to 'lock in' to either route, I don't get any warning dialogue like with Melvin and Garcias about route locking.
Edit: also worth noting that the first time I went down the Loyal Melvin route and left the Sato route hanging, I ended up with the normal route, despite having "canceled" the Garcias route and getting the affirmative dialogue from Adriana that she was still friends with us. There was also specific dialogue and scenes throughout that playthrough (Melvin meeting both Adriana and Fujiko, Melvin brining up a foursome at one point), so not sure if that's typical of a normal harem route or if something bugged up along the way.
But shortly explained you have to say no to drinks at the club with sato(It's getting late, go home) very early on in his route - That's the trigger you are looking for.