> So like a love hotel (http://en.wikipedia.org/wiki/Love_hotel), but sans coupling. Or like a capsule hotel (http://en.wikipedia.org/wiki/Capsule_hotel), but with attached shower.

To be fair, they say Japanese love hotels allow single guests (https://en.wikivoyage.org/wiki/Japan#Love_hotels) and capsule hotels have showers (https://en.wikivoyage.org/wiki/Japan#Capsule_hotels).

I don't doubt, though, that even in Japan there's a niche for specifically targeted establishments of the kind described by grandparent.