Argh, of course right after I posted this question I figured it out! Sorry about that.
For posterity's sake, if anyone is curious:
Although the 2dtk Fonts don't seem to have a "hard edges" option, you can manually open the font texture in Unity (i.e., the .tga you generated in BMFont and imported into your project), and then change the Filter Mode for the texture to Point. Voila, hard edges!