[David]
Has anyone actually ever used those available bits for the zillions of NaNs for anything good?
Yes: in Python, many sample programs I've posted cleverly use NaN bits to hide ASCII encodings of delightful puns ;-) Seriously? Not that I've seen. The _intent_ was that, e.g., quiet NaNs could encode diagnostic information, such as the source code line number of the operation that produced a qNaN. But I don't know that anyone ever exploited that. Signaling NaNs were even more quixotic. For example, in theory, an implementation _could_ reserve some range of sNaN bit patterns to mean "the lower 20 bits are an index into a table of extended precision values", and a trap handler could catch the signal when the sNaN was used, and do extended-precision calculation in software, store the result in the table, and return an sNaN containing the result's index (or a regular double if the result fit in the format). In short, the kinds of things hardware designers think software would love ;-)