Having fromhex() support signed NaNs was reasonable because it is a valid possible input; however, the case isn't as clear-cut for output. Since applications are allowed to ignore the sign bit, no application should rely on seeing a particular sign for a NaN.  

Also, existing applications may already have NaNs where the sign bit is set.  If we change the output, we could break their published examples and doctests.  So, this proposal may cause more harm than good.