In ECMAScript, the set of values represents the double-precision 64-bit format IEEE 754 values ...

Yes, it seems to me that there would likely be people who want "double-precision 64-bit format IEEE 754 values", irrespective of perl's NVTYPE.
However, for long double builds, JSON::XS currently presents "long double" values, so having quadmath builds present "__float128" values is at least consistent with that.

I gather that JSON::XS is essentially abandonware (and no-one uses quadmath builds, anyway), but I've placed the patch I've used at https://rt.cpan.org/Ticket/Display.html?id=155704.

Thanks kikuchiyo, afoken, for your help.

Cheers,
Rob