After some investigation, I'm sorry to say, that you've already found the answer to your question.
Deal with it, there are no better news.

As I understand, it isn't a problem of DXF version. It is a problem of how OS X opens files with associated application. On Windows and Linux the filename is passed as command line argument to LibreCAD when the DXF file is double-clicked. OS X has a completely different approach, it uses an open app which calls the associated app, but doesn't pass any arguments to it.

But, you will say, other applications can do this and you are right. The problems are, we don't have an OS X developer who possibly can solve this easy and we are limited in time resources, we all work in our spare time on LibreCAD.

I've created a feature request on sf.net to keep this in mind.

Until then the way for OS X is to use the LibreCAD file open menu.

Armin

---