@DSquared125 - I don't believe so. This plugin uses calibre code to locate fonts on the system, which in turn uses a third party 'fontconfig' plugin if what I saw in the code is correct. Perhaps Kovid can chime in if he spots this. My code uses this:
self.font_family_model.families = fontconfig.find_font_families(allowed_extensions=['ttf'])
The only thing I could "possibly" do is change what extensions are searched for - as you found I currently only allow for .ttf fonts. This was code I copied from elsewhere in Calibre. I don't know anything about Mac OS fonts (or fonts in general) to know what the impact of trying to support a .dfont (or indeed whether that fontconfig plugin supports them), so it isn't a change I would rush into without someone confirming the impact.