I have a Harmony 890 and can confirm that it can control multiple Z-wave light switches by grouping them into scenes which you can then assign to "activities" like "watch movie" or "listen to music". This is a nice feature, but not very useful in my opinion.
You CANNOT on the other hand assign the different scenes to buttons as the individual dimmers or scenes are not treated as different devices with the harmony remotes. i.e. you cannot assign a scene like "all lights off" to the play button in your "watch movie" activity or a scene such as "all lights 50%" when you press the pause button to go to the bathroom or something. In order to control the scenes, you have to exit the current activity, then go into "Z-wave device" or something like that, then go into "scenes", then press the scene you want! Way too many button presses for me.
I have contacted Logitech 3rd tier support or something to that effect and they currently have no plans in the future that I know of to improve the functionality of Z-wave controlled devices beyond what is now possible. The tech support person even agreed with me that I would have been better of by purchasing IR controlled dimmer switches. Sigh!!
I hope that someone reading this can tell me that I am wrong...but for all of you considering Z-wave switches for your Harmony 890 or 1000, think twice before you buy!
Hope this saves some of you the trouble that I went through.