I think healthy debate can be had as to how open ZUN would be to lending his characters to smash. On the one hand ZUN has traditionally not wanted to associate with the more mainstream games industry, he partly started Touhou to get away from that after all, and Smash in some ways feels like the epitome of that. On the other, ZUN has been increasingly lax with Touhou in recent years, even more than it has always been, from official games on Steam, PS4, and soon Switch, to Touhou characters cameoing in Taiko Drum Master on 3ds alongside Kirby, and most recently Cannonball; ZUN used to be very against the idea of a Touhou gacha, but obviously he changed his mind. Ultimately, I think ZUN would give the go ahead, but I don't blame anyone for thinking he wouldn't.