I’ve updated the Focus Mask plugin with pick/reject buttons, which are very handy way to get rid of out-of-focus images.
Here’s my own workflow with Focus Mask. First, simply select the photos (shift-click, up to 32) and start Focus Mask plugin.
Click “Decrease Flag Status” button so that all the photos are in rejected state. Now raise the photos you see as good enough focus by clicking on “pick” or “unflagged” radio buttons (on Mac, you can also click the photo itself, its flag status is then increased). Clicking OK will make the pick changes permanent.
This plugin is only compatible with Lightroom 4. If you are running LR3, I could perhaps modify plugin so that a subset of the functionality would be available on Lightroom 3.x versions, but yet there has not been significant interest for that!