I honestly don't think Nintendo would be willing to use SNW for HHN at all; if Marvel didn't like how its IPs were treated during HHN, I think Nintendo will absolutely hate it. The level of scares HHN sets the bar for is something Nintendo wouldn't touch with a 39 1/2 ft pole. Knowing how protective Nintendo can be about their IPs, Halloween events/decorations/costumes for the characters during regular hours would be as far as Nintendo would let them go, and if anything to the tune of Luigi's Mansion were to happen, it would happen during that time frame of the day.
Unlike Wizarding World, which has HHN potential with the Death Eaters like we saw in Osaka, I don't see SNW opening during an HHN event at all. Even with that teen/YA audience that connects with Nintendo as well, as far as we know, Mario and DK are not IPs Nintendo would let Universal venture into the horror genre with. But that's not to say a Halloween SNW event in the daytime wouldn't be special; in fact, that idea alone would be special enough. There's definitely potential for a daytime Nintendo-themed Halloween event. Nintendo is a brand that appeals to all ages, and I'm confident they can come up with something awesome in that regard during regular hours. But for HHN? Forget it, not happening.