To the best of my knowledge there is no power flow calculation with a distributed slack bus in matpower. I am thinking about making a contribution in this regard. Is it enough to implement a power flow with distributed slack bus in only one algorithm (e.g. newton (power, polar))?
At my own fork of the matpower repository I created a pull request with a concept/idea on how to implement a power flow with a distributed slack bus. I would appreciate it if you have a quick look and give me some feedback.