You need an addon which is stored somewhere not in standard addons directory.
- Change Scripts path at user preferences ("File" tab) to somefolder
- Activate Add-ons tab
- Push Refresh button
- Find your addon
- Try to enable it
You will see "addon not found: your_addon" in console.
This is because sys.path is being set up only once at blender startup. So, unnecessary Blender reload is required to install an addon from user-defined scripts directory.
Fix is attached.
Alexander Romanov (Blend4Web Team)