I currently have it working on top of weston/wayland.
It runs Qt/QML using QtQuick2ApplicationViewer. Previously, in my case, on another platform without a window manager, the GUI part of the application was started using QtQuick2ApplicationViewer::showExpanded(). This was causing window decorations to be drawn when running in a window manager like Weston.
Changing this to QtQuick2ApplicationViewer::showFullScreen() made the difference for my case. Eventually, I need to make a custom splash screen that is used prior to Application startup (for the entire boot sequence u-boot, Linux, Weston, etc.)
If you are running Qt with QML, this should work for you. Not sure how this would translate to using Qt with Widgets, in case that is the method you are using.
it's not what I originally set out to do, but if the Weston background wallpaper could be changed to use the same screen as the splash screen in u-boot, Linux, it might work well enough. That way, on boot, I see the splash screen and then the application full screen, and on shutdown, briefly see the splash screen again, which is what I want.
Best,
marion