I think the window for a JJverse Shatner appearance to really work has already passed. Specifically, the scene in Trek XI that was written for Shatner
which never happened. That would've been absolutely perfect
and I very much doubt that anything else they try will hit home with the same force. Better to leave it alone than half-ass it.
On the other hand? Shatner is an octogenarian now. If they're going to do this, it needs to be as soon as possible.