I’ve spend way to many hours goggling this. Hopefully someone in need this will find this post before wasting 9 hours on figuring out how to do something this simple, like I had too.
What I did to get to this, was fire up SQL Profiler and look at the SQL being called while disabling and enabling a system user. After messing around inside sdkmessage and sdkmessageproccessingsteps I finally got a few hints and while goggling I stumbles over this page, and gave it a try. And it seemed to work.