You can also the Select command (Ctrl-S by default) in Advanced mode to highlight files containing certain text. Similar to using the Find tool but may or may not be preferable. You can modify the select command so that it hides files which don't match, too.
Not sure if there is a good way to jump to the "Contains" clause in a filter via the keyboard so I think both these solutions will involve a bit of mouse clicking and aren't the pure keyboard solution you're after.
Of course, you can assign a hotkey to toggle the viewer pane on and off if you want to. It's the Find/Select part which isn't ideal from the keyboard.