Modifying a perspective's focus
 

I've created a perspective that utilizes the focus capabilities
(specifically to Projects) in OmniFocus. If I'm currently in said
perspective, how can I modify the perspective to include another project,
without having to completely respecify, via unfocusing and reselecting, all of the projects I'm trying to
limit to?

Not currently possible. As you guessed, you'll need to un-focus, select the ones you want in the new version, then re-focus. I'll be happy to file a feature request on this, though. Thanks!