Although getting here was pretty time-consuming, the end result is simple. I did this on Windows but it should work on a Mac as well, provided you can install Pillow and run some Python.
- Connect the iPhone to your computer
- Backup the iPhone using iTunes
- Grab iExplorer and run it, allowing it to connect to the iPhone
- In the Backup Explorer, navigate to HomeDomain -> Library -> Springboard
- In that directory you'll see HomeBackground.cpbitmap and LockBackground.cpbitmap files, along with .jpg thumbnails of the files. Export these files from iExplorer to your computer.
- The .cpbitmap files are worthless (except to the iPhone) on their own, so finally you need to run a Python script to convert the .cpbitmap files to a usable image file.
Hope that helps others retrieve their wallpaper files!