Ah, sorry, I didn't check that. Just assumed the snippet above worked. OK, that makes things a bit more complicated.
A simple approach like this mostly works:
file = '/User/Default (Windows).sublime-keymap'
command = 'some_command'
json_data = json.load(open(sublime.packages_path() + file))
command_map = [key for key in json_data if key['command'] == command]
However, the Python json parser does not allow for comments, which are used in most default files. To handle that, you need a custom decoder object.