ffmpeg re-encode libx264 without quailty loss? According to the documentation for libx264 (h264) encoding, we can use a preset of veryfast, faster, fast, medium, slow, etc.
https://trac.ffmpeg.org/wiki/Encode/H.264


  ...for constant quality encoding, you will simply save bitrate by
  choosing a slower preset.


So while using constant quality, a slower preset doesn't have any affect on video quality, but it can compress the file size smaller.

This theoretically means it should be possible to take a video compressed with preset "veryfast", and recompress it with "slow"- resulting in a smaller file with no loss of video quality.

Has anyone ever done this or know how it can be done?
You have a misunderstanding of how compression works.  In all but a few specialized types of lossy compression, when you compress something a second time, even in a much higher quality level than previous encodings, you still lose additional quality.  

Using a slower encoding from the same original source with constant quality will often produce a smaller file of similar quality, but if you encode the already compressed version over again, you will still always end up with a lower quality video due to second generation quality losses from compressing twice.