The multi-dock is pretty useful for housing your plugins so you don't have to open and close them, the Piano Roll, and of course for housing the Console. Navigating between the different multi-dock items is simple too, Ctrl+Shift+Left/Right. Most of the time you'll have something open in the multi-dock one way or the other. Get used to keeping it open and navigating between windows so you don't have to frequently open and close them, and you'll have access to Melodyne the way you want.

Thanks for your suggestions, everybody!
 
I think they all don't fit my workflow though, for various reasons, so I'm afraid I'll have to live with that situation. Sonar isn't my primary DAW, so I definitely can live with it, but it would be nice to be able to configure window default sizes and positions independently of the docker.
 
I do want the docker to stay on the first screen (to display the region list below the arranger view), while Melodyne should always open on the second screen and always in maximized mode. That does not seem to be possible.
 
Leaving Melodyne open all the time is a workaround that I'm not comfortable with when doing a complete mix, but for a pure editing session, it would do the trick.

instead of using Melodyne in the docked window, put it in the FX bin by right clicking the FX bin and then choosing VST3>Celemony>Melodyne
 
then your melodyne window is independent of the sonar windows, just like any other VST. 

When you use Melodyne as a Region fx it is located in the MultiDock. If you use it as a VST you lose the region fx workflow which is the point of the tight ARA integration.

mudgel
When you use Melodyne as a Region fx it is located in the MultiDock. If you use it as a VST you lose the region fx workflow which is the point of the tight ARA integration.

I understand that. however, IMO the "region FX" is not that big of a deal when using melodyne.  you can always transfer only a region of the track to melodyne when it's in the FX bin.

John T
The multi-dock automatically closes when there is nothing in it. The way I get around this is that I always have the console view in it, and I keep the dock full screen on the second monitor. That way, if I've got a melodyne window open in the multi-dock, and I bounce the clip, the melodyne closes, but the dock doesn't.

+1