The Mumble overlay is only visible in EVE Online while it is starting (and its window is black). Once the EVE login screen appears, the Mumble overlay stops working.
1. Configure the overlay to always show channel members. (this makes it easier to see what's happening with this bug)
2. Start EVE online. Notice that the overlay is visible while the screen is black.
3. Once the EVE login screen appears, the overlay isn't visible any longer.
This is happening with Windows 8.