Add target attribute

Now and then one has a need to have a menu item that loads in a new window. A few examples:

• A file (PDF, Word doc, etc.) that will behave in an unpredictable way.
• A sub-site or major sub-section in your site that you'd like users to have open in a separate window
• Simply adding links to resources that are not within Drupal's control

Surprisingly enough, nothing else does it. You're going 80% of the way there already. It's a logical fit in terms of one way of thinking of what you're doing ("filtering link properties"), though clearly not what you originally had in mind.