I'd be inclined to re-joint the back and put a tapered strip in. Heck I'd probably use ebony or something!
If the rest of the joint is already solid though, I guess you could inlay a slice into the gap - but it wouldn't look as nice and probably wouldn't be as solid long-term...
Basically I'd un-do everything and make the back perfect, then go from there. Since you have the top and back both off (right?), getting the back to be a nice perfect platform should be a lot more pleasant when you're putting things back together!