I’m not certain there’s any established Japanese naturism. If there is, I’ve certainly never heard of it. What there is, though, is onsen -hot springs baths- which were apparently mixed sex and nude (the whole family went) and while bathing rituals continue, they’ve apparently become more single-sex, particularly in urban areas.
Bathers in an onsen. Note that both are wearing towels IN the water
The sento is another word for these bath houses, which sometimes allow the carrying of a small towel for ‘modesty’ while walking between the changing area and baths, but it is considered unclean to immerse that in water, so bathers fold the towel and place it on their heads.
I would suggest that those who visit the onsen available in SL maybe take their cue from any Japanese users of the baths with regard to dress code, and follow their lead accordingly. While not strictly naturist, there are opportunities for ‘clothes optional’ play in SL in these baths.
Almost all of SL’s onsen offer free towels for wear in and out of the water. I’ve picked up copies of these where I can, because they can also be utilised in other, non Japanese naturist environments. It’s not untypical to see naturists wrap themselves in towels on exiting the sea, to dry themselves off before returning to their sunbathing, and I’ve worn these onsen type towels within several naturist sims inworld.