If you get no context menu at all for zip files then try going through the steps in this FAQ to see if a shell extension is breaking it:
Crash, exit or high CPU usage when right-clicking certain files
You need to fix what happens when you double-click Zip files outside of Opus first. i.e. Make WinRAR (or whatever you want) take over the file type. When you double-click something in Opus, and the Left Double-Click event is not overridden, then the result should be the same as double-clicking something outside of Opus. If the result is wrong outside then it'll be wrong inside as well.
Hmm, that could be a problem. 
Perhaps Opus needs an advanced "don't repair Zip filetypes" setting to prevent this. What it does makes sense in most cases (if you want Opus to handle zips at all then you probably want double-clicks on them within Opus to open them in Opus) but it should be possible to override it.