I finally got around to building and running the JUCE demo for x64 Windows. It works fine with only minor changes:
-The Ogg Vorbis code contains inline assembly, so I had to turn off JUCE_USE_OGGVORBIS.
-In juce_win32_FileChooser.cpp, I had to change the return value of openCallback (line 117) to a UINT_PTR instead of an unsigned int.
And that was it. The demo app builds and runs fine as a native x64 app.
So, you know, nifty.