I have developed an example plugin that adds the Regenerate Thumbnails actions to the Media/Assistant item-level rollover actions and to the Bulk Actions dropdown controls. When any of the new actions is invoked the Regenerate Thumbnails screen is displayed and the action is executed. When the action completes you can click the link on that screen to return to the Media/Assistant submenu screen.
I have uploaded a new MLA Development Version dated 20160913 that contains the example plugin. To get the Development Version, follow the instructions in this earlier topic:
Shortcode not working in (special) widget
After you install the Development Version you can easily install the example plugin. Navigate to the Settings/Media library Assistant Documentation tab and click the “Example Plugins” button. You will see a new submenu that lists all the example plugins and give you a “one-click” action for installing them.
You are looking for “MLA Regenerate Thumbnails Example” plugin. Find that plugin and hover over the title in the left-most column. Click the “Install” rollover action, then go to the Plugins/Installed Plugins submenu and activate it.
Once the plugin is active the actions are added; there is no other user interface for the example plugin.
The Development Version also has a completely re-written Settings/Media Library Assistant Shortcodes tab with a more WordPress-like interface. I welcome any feedback you have on the new implementation.
I hope this works for your application. If you have problems or further questions, post an update here and I will help you get the results you need.