I wasn't able to find really good information about whether a load/store of a signaling NaN will cause an exception.  Some documentation reads as if the exception would only be raised after an arithmetic operation.  The best information I could find was:

http://lists.freebsd.org/pipermail/freebsd-numerics/2012-August/000124.html

In my testing I am using x86-64, so I don't think it would raise.  However, it seems like the handling of signaling NaNs in general is very processor/architecture/instruction extension dependent.  I agree that it is a better idea to work first in non-FPU to swizzle before loading it into the FPU.  I will rearrange the v8_typed_array_bswap code to allow for load/store.  This should generate code that will move from the FPU to general registers (or the other way around) and do swizzling there.  This is almost already the case except there is a unnecessary bounce back and forth, and this will eliminate that which is a good thing anyway.