As the title says, it would be useful to have an option on the panel settings to make it so it shows in the overview mode. This would behave similarly to gnome's activity overview or macos mission control. The usecase for this feature is opening the overview just to find that the window you're looking for isn't there, so you can just click the launcher on your panel directly from the overview to launch it. This is also very useful for panels that autohide.
Note that this would require not just the panel to be visible, but also being able to interact with it. It's quite complex, I'm really not sure I'd like to see that :-/
Yeah, this would be quite a technical can of worms. Probably not feasible unfortunately. I think when you say you want to see the panel, it's possible that what you mean is that you want a program launcher visible in Overview. That's theoretically more feasible. It would require a lot of re-plumbing though, as we'd have to expose either the Kickoff/Kicker favorites or a particular Task Manager instance's pinned apps via a new separate library that both plasma-desktop and KWin could use. We couldn't grab the data right from Plasma without this library approach as this would make Plasma a dependency of KWin and make it impossible to use separately. I think it would make the most sense to expose Kicker/Kickoff favorites since those are global in scope already, matching Overview's scope.
*** This bug has been marked as a duplicate of bug 460437 ***