I made a butler.bat file that just calls:
C:\Users\<username>\AppData\Roaming\itch\bin\butler %*
Which is used by the upload script for my games, which call it like:
SET PROJ_SERVER=my-game
SET PROJ_EXE=MyGame
SET BUTLER_EXE=..\butler.bat
call %BUTLER_EXE% push build\%PROJ_EXE%Windows gatechgrad/%PROJ_SERVER%:win
However, in the new version of the Itch app (25), the location of butler was changed, so now I have:C:\Users\<username>\AppData\Roaming\itch\broth\butler\versions\15.6.0\butler %*
The problem is that my butler.bat will need to be updated each time a new version of butler is released, since the version number is a part of the directory structure now. Unfortunately, since I'm on Windows I can't just create a symbolic link. However, it's better than having to compile butler from source myself. There is a ".chosen-version" file, which probably could be used with some scripting magic to find the correct butler folder to use. I'm already writing a graphical Unity build tool in Ruby (https://bitbucket.org/gatechgrad/unityhelper/src/default/UnityVersionDisplay/), so it probably wouldn't be too difficult to just go ahead and include an Itch/butler upload function as well, and just be done with the bat files.