I figured out my problem! The issue was the path I was providing wasn't being string-manipulated properly. The sample code I gave above was what I expected to be there. Oops!
BTW, the code looks familiar to you because you posted it somewhere else, and that's what I ended up using, so thanks a lot for that!
One other thing though... What's the best way to deal with the fact that Android doesn't display the files in the gallery until the mediastore gets refreshed? Since I'm copying these assets in right before the test, I'm not sure what the best course of action is.
When running locally, I know that you can simply do:
adb shell am broadcast -a android.intent.action.MEDIA_MOUNTED -d file:///sdcard
But my understanding is you can't execute adb commands on the emulator when running Appium remotely...