So, I tested running wayland compositor inside qt-runner and … it works. Tested using pure QML compositor shown as an example at qtwayland/examples/wayland/pure-qml at 5.15 · qt/qtwayland · GitHub
As a result, it was possible to run apps inside that nested compositor and it allowed to use newer Wayland protocols. The drawback that I saw was a loss of WebGL in QtWebengine, there could be more. But, in principle, as long as compositor is from Qt - Qt is the one supporting the old protocol - it is possible to get newer compositor protocols supported too.
It is even possible to run Silica apps inside that compositor. Just there are some odd aspects, such as transparent background.