This was actually super easy -- Kayak.de to Expedia -- the trick (for Japan in general) is to book way out, when the hotels have loaded nothing but ridiculously high rates. My actual stay dates had rates of ~25k yen on SPG.com, and while you'd likely still be able to find a lower competing rate, you'd probably end up in the 20k range after discount. My most successful BRGs are booked 8-10 months out.