Notational Velocity (well, the version I tried, at least) has some Applescript support, but that support doesn't appear to include getting the currently selected text. Without the ability to do that, a clipping plugin for NV is going to be difficult to do in a straightforward manner.
I did come across a posting here:
http://www.macdrifter.com/tag/applescript/
that described a method for turning NV notes into OF actions, though personally I think editing the note to be easily identified as an OF action in the making is more work than simply selecting the text to be copied! Aren't you just talking one click plus cmd-A before the clipping shortcut to grab the text of the note?
If you are taking a note that is going to go into OmniFocus, why not just use the Quick Entry shortcut to bring up the Quick Entry box and dump it into OmniFocus in the first place? You don't have to assign a project, context, due date, etc., just capture that thought and process it later. I use OmniFocus as my ubiquitous capture tool, even for things that won't be actions. You can easily drag an action from OmniFocus into NV's search field, click in the text area and start typing if you want to go that direction.