Slow dimming is a funny thing… The Switch Level capability (common to all dimmer switches) has a command setLevel(int,int) where the second parameter is the duration of the change. If that was the end of the story, it would be relatively easy to extend Rule Machine to pass an optional parameter for timed fades. Unfortunately, it’s not the end of the story. It seems that the duration feature is only supported by some dimmers (since it must be implemented in the device firmware) so it may or may not work for your application.
Perhaps @bravenel is feeling feisty and wants to add experimental support for this parameter, but I wouldn’t blame him if he wanted to avoid the potential for bug reports which he has no control over (though this wouldn’t be new… many thanks to Bruce for being patient and providing the “Killer App” ST has always needed, despite the problems outside his control!)
A totally different option for slow dimming is a custom smart app. Here is an example from a pretty cool developer which would work with any dimmer. I’d say that this approach is too complicated to embed directly in Rule Machine, but could be made to work with some effort.