This is a simple app that uses the Muzei 3.0 API to save the currently displayed Muzei wallpaper to your downloads directory.
Step 0: Obviously, your current wallpaper should be Muzei 3.0+.
Step 1: Tap the icon in the app drawer or use the quick settings tile.
Step 2: Select where to save the image.
Step 3: Profit? Nope, because this app is FREE!
Hint: Power users (like me) will associate this app with a gesture, perhaps via Nova Launcher.
Free and open source. Download the code here: https://github.com/banasiak/MuzeiSaver