Regardless. If the author doesn't want anyone messing with his add-on it's a no-go, abandoned or not. I know I sure as hell wouldn't want people changing an add-on I made if I ever found I couldn't devote time to it.
Just because it hasn't been updated for 5.1 right away does not mean it's abandoned. It was updated for 5.0.5, so no reason it won't eventually be updated for 5.1. Or you know, private message the author of the addon on the ElVUI forums and ask him directly like i suggested.