So I found the problem, although I'm not sure quite what to make of it. Installing Chef Workstation didn't help, but it's because I had some leftover files in my ~/.chef directory (specifically, plugin_manifest.json) from a "knife rehash" run. And apparently, that file somehow managed to cause this error; I'm guessing it's overloading or overriding the module somehow. If you'd like to get a copy of what mine looked like for further troubleshooting, please let me know and I'll send it your way.
Removing that was sufficient to make knife start behaving when it comes to installing supermarket cookbooks.
Once again, thanks for the quick responses and assistance!