Hi Matt,
Seems I have found the solution for you on this page:
http://fontforge.github.io/fontinfo.html
Look for "Preferred Family" explanation in TTF Names section:
"This is to get around a quirk of windows where only four Style names are allowed per family, so font families with more than four styles would get different family names above, but the preferred family name would be the same. This should only be specified if it differs from the family"
In fact TTF fonts in AutoCAD for Mac handled the same way as in Windows version: you can see only four Style names per family, so in order to be able to use other typefaces (Styles) such as Light or Extra Bold you need to modify existing TTF files by changing "Preferred Family" property of the font.
You can do this in commercial application such as Glyphs, but you can also use FontForge application:
http://fontforge.github.io/en-US/downloads/mac/
Open your TTF font in FontForge.app, save it as FontForge source file (.SFD), go to menu Element -> Font info:

Than open TTF names section and edit "Preferred Family" field by adding new information to it, for example for Light font face you can add Light to the preferred name:

Save .SFD file and use menu File -> Generate fonts to generate new TTF file:

Now if you put this new file to the Fonts folder on your Mac you will be able to use this typeface in AutoCAD for Mac.
Maxim