RapDialer is just a dialer app, I doubt the developers have even thought of updating the tile for missed calls. RapDialer and the other dialer apps are not integrated into the phone system like that to be able to show things like missed calls, accept calls, or have a joint history. MS may not even allow apps to have access to those types of things. It's also not easy to have a tile which updates on demand like that, there are very few that do apart from the built in functions.
I guess the short of it is I wouldn't hold my breath.