Double-click the task bar space(not task bar icon) to activate the Directory Opus
There's no way to do that at the moment.
However, there is an option to open Opus by double-clicking empty space on the Desktop.
Hope that we can consider this function, the task bar blank space can be seen anytime, anywhere.
Unless you've got lots of windows open. 
Feature requests should be sent to GPSoftware: gpsoft.com.au/Support.html
Absolutely. I never have any spare space on the toolbar.
It doesn't achieve the same thing that the original question requested, but as a keyboarder by preference I find a keyboard shortcut to be a very quick way of launching anything.
If you have DOpus set to replace Explorer, "Winkey+E" is as fast as it gets.
Much quicker than faffing around with a mouse and the "Notification area" as we are now supposed to call the "System Tray".
Thinking about it, is the Preferences bit in DOPus using the right terminology? It says "From the Taskbar icon". Shouldn't this be "System Tray icon"?
DOpus would not be alone in this confusion. I recently found the same misuse in some Logitech software.
What most people call the "system tray" has always been called the "Taskbar notification area" since it first appeared in public in Windows 95:
blogs.msdn.com/oldnewthing/archi ... 54831.aspx
Whether many people know the correct name, or would be more likely to be confused by it, is another question.
"Taskbar notification area" is hardly the same thing as Taskbar, especially when the original user refers to "Taskbar space".
If that has always been the official title, it is even more puzzling for DOpus to use an incorrect label.
Hardly the end of the world, but inconsistent all the same.
The Taskbar Notification Area (AKA System Tray) is part of the Taskbar.
The phrase Opus uses, "Taskbar Icon," means an icon on the Taskbar. There are three sets of icons on the taskbar: The Start button, the icons for each window, and the icons in the Notification Area. I imagine most people will understand that it means the Notification Area icons and not the others but if anyone was unsure then 2 seconds of experimentation would resolve it for them.
Elsewhere in Preferences, Opus calls it the "Taskbar Status Icon." Not the official name for it but I'd say still obvious.
Perhaps Opus should call it the "Taskbar Notification Area Icon" in both cases. I'm not sure if that would confuse people more or less. 
TBH, I doubt anyone is confused by the name in the first place.
In this case you are probably right. Not much chance of confusion if you are an experienced user.
I spotted it because of the Logitech use that I mentioned earlier. That really was confusing. They really did talk of the thing docking itself in the Taskbar. Indeed, in efefct they offered an option to put the minimised program in either the Taskbar or, hmmm, the Taskbar.
As with all of these things, it is usually better to get it right. Old hands who have been using windows since, er, 3.0, will have plenty of deadwood in their brains. But newcomers have their brainboxes less polluted by leftovers from CP/M and the like. I mean, they are probably happy with "folders" as a label for what we all grew to love as "directories".