jjohns,
I am not sure what you mean by a “search tag” feature, but I can give you some ideas for finding Media Library items by taxonomy terms, e.g., tag values.
The available taxonomies are listed in the screen/subscreen list, between Media/Library and Media/Assistant. If you select one of these subscreens you will see a list of the terms in the taxonomy. There is a “Search” box in the upper right of these screens you can use to filter the list of terms. In the “Attachments” column, there is a count of the items that are associated with each term. If you click on the number in that column you will get a Media/Assistant screen showing all the items having that term.
On the Media/Assistant screen you can display a column for any of the available taxonomies. If you don’t see the taxonomy you want, pull down the Screen Options (upper-right corner) and check the box next to the one you want. Each of the terms in the taxonomy column is a link; click on a tern to get a list filtered by that term.
I hope that gets you started. If you have any problems with these procedures or further questions, let me know. Thanks for your interest.