Thank you for that link, jib71!
It seems that 2016 was a better year than usual for snake bites in Okinawa (the annual figure for the region is usually about 90-100.

Best bit in that first link was the part about snake distribution. Turns out the main "habu" isn't found on Zamami. We're visiting the terrain of her Royal High es, Hime Habu (the Okinawan Pit Viper). Apparently the Hime habu has a weaker venom for which anti-venom isn't produced. Still something to be feared and respected.

The HexArmor Superfabric sleeve arrived earlier today, I've already unpicked and disassembled it. Tried spearing myself with some Japanese scissors and there was no penetration (although a fine needle slips through with no problem). It's a good protective layer, strong, light, breathable, doesn't fray. I'll measure LapChild's legs later and then sew on some Velcro, won't need much work. (Total cost was £13.50 - just under 2,000yen - all in, they usually cost about £70). In short, LapChild will have some budget gaiters made from top of the range materials and mama is even less likely to be spending the rest of her life regretting the decision to take her hiking in the sub tropics.