Hi !
First, thanks so much for your work. It saved me hours ...
I just spent time to understand why your plugin wasn't working on some of my production websites and finally got it.
'update_core' capability makes the admin menu disappear when using WordPress DISALLOW_FILE_MODS constant.
Maybe 'export' would be more appropriate, since no core files are updated by the plugin.
It's just a suggestion ...
Thanks again,
Jeremie