I would expect one can export a constexpr function from a library (that seems necessary to me if one wants to write (most of) the C++ standard library in C++).

If so, a workaround would be to hack together a library that exports a constexpr function that converts floats to their bit representation (that might require assembly or binary editing of a library containing a function that does that by casting) and calling it from the C++ constexpr. Such a workaround would support negative zero and NaNs.