Thanks for your question, which came up a while back in this earlier topic:
Searching on custom fields
That topic inspired the creation of the “MLA Custom Field Search Example” plugin, which is included in the current MLA release. I just ran a quick test and discovered that the plugin works fine in the Media/Assistant Search Media box but does not run in the “Add Media” popup window (Media Manager Modal Window). I found and fixed a small bug and have an updated version on my system. I would also like to make some improvements such as making it easier to set a default custom field name.
In the earlier topic I describe why adding a checkbox would be impractical and why I took the approach I did. Have a look at the topic and, if you like, try out the current version (in the Media/Assistant screen). If you think this will work for your application I will make a few improvements and get you a version that works in the Add Media window.
I will leave this topic unresolved for now. Thanks for your interest in the plugin.