I’m mostly just placing this snippet of code here because I don’t want to lose it. Regardless, if you’ve been searching for a way to filter your list of posts (or CPTs) by a custom taxonomy (like you can filter through posts with native Categories), then this is your solution. It began with a WordPress.org forum post, which while close, didn’t quite get the job done.
After a bit of tinkering, I know I’ll be using this filter quite a little bit. So, bon apetite. Please note that I use an object-based functions.php file, so this is a snippet directly from my object. Your mileage may vary.