Glad to hear I'm not the only one that's experienced this issue. Indeed, I'm going to wait until late July and call in to try to book. Right now one-ways to Sapporo are pricing at around $300, which is ridiculous imo. 4500 Avios + $0 has a much nicer ring to it.
I will also look into those Yokoso fares if there's no availability (though I doubt it given how much open space I've been finding on JAL flights).