Comment 2 for bug 1816274

You can already do what you want.
1) Get a saved search expression into the Search box
1a) Click on a saved search in the tag browser
1b) Click on a saved search under the "Saved Search" button
1c) type it in
2) Click the "Saved Search" button
3) Click "Get saved search expression"

However, adding a "Search using saved search expression" action to the context menu is a good idea, so I will do that.