JNA now supports 64-bit windows. The the build is a bit nonstandard, but it's automatic and reproducible. I was hoping to use mingw-64, but that project is not yet sufficient for JNA's purposes. Fortunately, the MS has a free 64-bit cross compiler which does work.
Thanks to the fine folks at Aquafold (actually mostly Niels G. the CTO) for making it possible.
Thanks also to T. Heller of python's ctypes for the initial glue bits.